Add conversation-history instrumentation tests for LlmModule

Summary:
Adds `LlmModuleConversationHistoryTest`, an Android instrumentation test that exercises the multi-turn / KV-cache plumbing on `LlmModule`. The OKR theme this enables is "Feature testing → conversation history" (3.2), which depends on `prefillPrompt` + `resetContext` semantics being correct.

The test runs on the existing TinyStories-110M fixture pulled by `android_test_setup.sh` from the public `ossci-android` S3 bucket, so it works on **both** internal fbsource Android CI and OSS GitHub Actions Android CI without any new fixture infrastructure.

Because TinyStories is too small and not instruction-tuned, content-level assertions (e.g. "did the model recall the user's name") are not reliable. Instead, the test asserts four behavioral invariants of the conversation-history surface that any production multi-turn flow depends on:

1. `testResetContextProducesDeterministicOutput` — at temperature=0 (greedy decode), running the same prompt twice with `resetContext()` between yields identical token streams. This is the foundational invariant: clearing the KV cache truly returns the model to a clean state.
2. `testKvCacheStatePersistsAcrossGenerateCalls` — without `resetContext()` between calls, two `generate()` calls with the same prompt diverge, proving the KV cache is preserved across turns. If this ever fails, multi-turn conversation is silently broken.
3. `testPrefillPromptInfluencesNextGeneration` — `prefillPrompt(history)` followed by `generate(prompt)` differs from a clean-context `generate(prompt)`, proving the prefilled context actually reaches the decoder.
4. `testResetContextClearsPrefilledHistory` — `prefillPrompt + resetContext + generate` matches a clean-slate `generate`, proving reset fully clears prefilled state.

Differential Revision: D105741356

Author: psiddh

extension/android/executorch_android/src/androidTest/java/org/pytorch/executorch/LlmModuleConversationHistoryTest.kt

@@ -0,0 +1,200 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+package org.pytorch.executorch
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import java.io.File
+import java.io.IOException
+import org.apache.commons.io.FileUtils
+import org.junit.After
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.pytorch.executorch.TestFileUtils.getTestFilePath
+import org.pytorch.executorch.extension.llm.LlmCallback
+import org.pytorch.executorch.extension.llm.LlmModule
+
+/**
+ * Behavioral tests for multi-turn / conversation-history semantics on [LlmModule].
+ *
+ * These tests run on the TinyStories-110M fixture pulled by `android_test_setup.sh`, which is too
+ * small and not instruction-tuned, so we cannot assert anything about the *content* of generated
+ * text (e.g. "did the model recall the user's name"). Instead, we assert structural invariants of
+ * the KV-cache + reset plumbing that any conversation-history feature depends on:
+ * 1. Determinism after [LlmModule.resetContext] at temperature=0 (greedy decode).
+ * 2. State preservation across successive [LlmModule.generate] calls (no reset → output diverges).
+ * 3. [LlmModule.prefillPrompt] influences the next [LlmModule.generate] call.
+ * 4. [LlmModule.resetContext] fully clears prefilled state.
+ *
+ * All tests run on both internal (fbsource Sandcastle) and OSS (GitHub Actions) Android CI because
+ * the fixture is fetched from the public `ossci-android` S3 bucket by `android_test_setup.sh` and
+ * the test only depends on the public `LlmModule` API.
+ */
+@RunWith(AndroidJUnit4::class)
+class LlmModuleConversationHistoryTest {
+
+  private lateinit var llmModule: LlmModule
+
+  @Before
+  @Throws(IOException::class)
+  fun setUp() {
+    val pteFile = File(getTestFilePath(TEST_FILE_NAME))
+    val pteStream =
+        requireNotNull(javaClass.getResourceAsStream(TEST_FILE_NAME)) {
+          "Test resource $TEST_FILE_NAME not found; did android_test_setup.sh run?"
+        }
+    FileUtils.copyInputStreamToFile(pteStream, pteFile)
+    pteStream.close()
+
+    val tokenizerFile = File(getTestFilePath(TOKENIZER_FILE_NAME))
+    val tokenizerStream =
+        requireNotNull(javaClass.getResourceAsStream(TOKENIZER_FILE_NAME)) {
+          "Test resource $TOKENIZER_FILE_NAME not found; did android_test_setup.sh run?"
+        }
+    FileUtils.copyInputStreamToFile(tokenizerStream, tokenizerFile)
+    tokenizerStream.close()
+
+    llmModule =
+        LlmModule(getTestFilePath(TEST_FILE_NAME), getTestFilePath(TOKENIZER_FILE_NAME), 0.0f)
+    llmModule.load()
+  }
+
+  @After
+  fun tearDown() {
+    if (::llmModule.isInitialized) {
+      llmModule.close()
+    }
+  }
+
+  /**
+   * resetContext() + greedy decode (temperature=0) must produce identical output across two runs
+   * with the same prompt. This is the foundational invariant any conversation-history feature
+   * relies on: clearing the KV cache truly returns the model to a clean state.
+   */
+  @Test
+  @Throws(IOException::class)
+  fun testResetContextProducesDeterministicOutput() {
+    val firstRun = generateAndCollect(PROMPT_A)
+    llmModule.resetContext()
+    val secondRun = generateAndCollect(PROMPT_A)
+
+    assertTrue("Expected non-empty generation on first run", firstRun.isNotEmpty())
+    assertTrue("Expected non-empty generation on second run", secondRun.isNotEmpty())
+    assertEquals(
+        "Greedy generation after resetContext() must be deterministic for the same prompt.",
+        firstRun,
+        secondRun,
+    )
+  }
+
+  /**
+   * Without resetContext() between calls, KV-cache state persists and influences subsequent
+   * generation. Generating the same prompt twice in a row should produce different output the
+   * second time (because the KV cache is no longer empty and start position is non-zero), or the
+   * second call may throw because the runtime detects the stale KV state.
+   *
+   * Either outcome proves state persistence. If this test ever starts failing (i.e. both calls
+   * succeed with equal output), the runtime is silently dropping state between generate() calls —
+   * that would break multi-turn conversations.
+   */
+  @Test
+  @Throws(IOException::class)
+  fun testKvCacheStatePersistsAcrossGenerateCalls() {
+    val firstRun = generateAndCollect(PROMPT_A)
+    assertTrue("Expected non-empty generation on first run", firstRun.isNotEmpty())
+
+    try {
+      val secondRun = generateAndCollect(PROMPT_A)
+      assertNotEquals(
+          "Without resetContext(), repeated generate() calls must reflect persisted KV state.",
+          firstRun,
+          secondRun,
+      )
+    } catch (_: RuntimeException) {
+      // The second generate() threw because KV-cache state from the first call
+      // affected execution — this also proves state persistence.
+    }
+  }
+
+  /**
+   * prefillPrompt() must influence the next generate() — i.e. prefilled tokens are part of the
+   * conversation history. If prefilling has no effect, multi-turn flows that rely on injecting
+   * prior turns via prefill are broken.
+   */
+  @Test
+  @Throws(IOException::class)
+  fun testPrefillPromptInfluencesNextGeneration() {
+    val baselineRun = generateAndCollect(PROMPT_A)
+
+    llmModule.resetContext()
+    llmModule.prefillPrompt(PREFILL_HISTORY)
+    val withHistoryRun = generateAndCollect(PROMPT_A)
+
+    assertTrue("Expected non-empty baseline generation", baselineRun.isNotEmpty())
+    assertTrue("Expected non-empty post-prefill generation", withHistoryRun.isNotEmpty())
+    assertNotEquals(
+        "prefillPrompt() must alter the KV state seen by the next generate() call.",
+        baselineRun,
+        withHistoryRun,
+    )
+  }
+
+  /**
+   * resetContext() must fully clear prefilled state — running prefill then resetting then
+   * generating should match a clean-slate generation of the same prompt.
+   */
+  @Test
+  @Throws(IOException::class)
+  fun testResetContextClearsPrefilledHistory() {
+    val cleanRun = generateAndCollect(PROMPT_A)
+
+    llmModule.resetContext()
+    llmModule.prefillPrompt(PREFILL_HISTORY)
+    llmModule.resetContext()
+    val postResetRun = generateAndCollect(PROMPT_A)
+
+    assertTrue("Expected non-empty clean run", cleanRun.isNotEmpty())
+    assertTrue("Expected non-empty post-reset run", postResetRun.isNotEmpty())
+    assertEquals(
+        "resetContext() after a prefillPrompt() must fully clear KV state.",
+        cleanRun,
+        postResetRun,
+    )
+  }
+
+  private fun generateAndCollect(prompt: String): String {
+    val collector = CollectingCallback()
+    llmModule.generate(prompt, SEQ_LEN, collector)
+    return collector.text()
+  }
+
+  private class CollectingCallback : LlmCallback {
+    private val tokens: MutableList<String> = ArrayList()
+
+    override fun onResult(result: String) {
+      tokens.add(result)
+    }
+
+    override fun onStats(stats: String) = Unit
+
+    fun text(): String = tokens.joinToString(separator = "")
+  }
+
+  companion object {
+    private const val TEST_FILE_NAME = "/stories.pte"
+    private const val TOKENIZER_FILE_NAME = "/tokenizer.bin"
+
+    /** Short prompt; SEQ_LEN kept small to keep the test fast on CI emulators/devices. */
+    private const val PROMPT_A = "Once"
+    private const val PREFILL_HISTORY = "Long ago, in a small village by the sea, "
+    private const val SEQ_LEN = 24
+  }
+}