feat: add ContainerEnvironment backed by the Docker CLI

Adds an JVM Environment that runs commands and reads and writes
files inside a long-lived Docker container, plus a demo agent wiring it into an
EnvironmentToolset.

PiperOrigin-RevId: 967183144

Author: wikaaaaa

core/src/commonMain/kotlin/com/google/adk/kt/annotations/ExperimentalEnvironmentApi.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.kt.annotations
+
+/**
+ * Marks ADK's **environment** APIs (execution environments and the environment toolset) as
+ * experimental: their shape and semantics may change in future releases without prior notice.
+ *
+ * Opt in explicitly with `@OptIn(ExperimentalEnvironmentApi::class)` to use them.
+ */
+@MustBeDocumented
+@Retention(AnnotationRetention.BINARY)
+@RequiresOptIn(
+  level = RequiresOptIn.Level.ERROR,
+  message =
+    "ADK environment APIs are experimental and may change at any time. " +
+      "Opt in with @OptIn(ExperimentalEnvironmentApi::class) to acknowledge the risk.",
+)
+annotation class ExperimentalEnvironmentApi

core/src/commonMain/kotlin/com/google/adk/kt/environment/Environment.kt

@@ -0,0 +1,133 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.kt.environment
+
+import com.google.adk.kt.agents.ReadonlyContext
+import com.google.adk.kt.annotations.ExperimentalEnvironmentApi
+import com.google.adk.kt.tools.ToolContext
+import kotlin.time.Duration
+
+/**
+ * The exception type wrapped in [Result.failure] by [Environment] operations.
+ *
+ * An environment wraps this in [Result.failure] for recoverable failures (e.g. a missing file, an
+ * unreadable path, or a failure to launch a command). The [message] is intended to be forwarded to
+ * the model as a tool error, so it MUST be precise and self-contained, and MUST NOT leak sensitive
+ * internal detail. Implementations should only wrap [EnvironmentException] in [Result.failure].
+ */
+class EnvironmentException(message: String, cause: Throwable? = null) : Exception(message, cause)
+
+/**
+ * Result of a command execution.
+ *
+ * @property exitCode The exit code of the process.
+ * @property stdout Standard output captured from the process.
+ * @property stderr Standard error captured from the process.
+ * @property timedOut Whether the execution exceeded the timeout.
+ */
+data class ExecutionResult(
+  val exitCode: Int = 0,
+  val stdout: String = "",
+  val stderr: String = "",
+  val timedOut: Boolean = false,
+)
+
+/**
+ * Interface for code execution environments.
+ *
+ * An environment provides the ability to execute shell commands, read files, and write files within
+ * a working directory. Concrete implementations include local subprocess execution, sandboxed
+ * execution, container environments, and cloud-hosted environments.
+ *
+ * The operations are `suspend` and may be invoked concurrently from parallel coroutines. This
+ * interface does no synchronization — when implementing an environment, keep concurrency in mind:
+ * overlapping operations can race on the shared working directory. [close] is not `suspend`, so
+ * that an environment can be closed wherever the ADK's other [AutoCloseable] types are.
+ *
+ * [execute], [readFile], and [writeFile] return a [Result] whose failure case is an
+ * [EnvironmentException] with a message intended to be surfaced to the model; implementations
+ * should only wrap [EnvironmentException] in [Result.failure].
+ *
+ * Lifecycle:
+ * 1. Construct the environment.
+ * 2. Call [initialize] before first use.
+ * 3. Use [execute], [readFile], [writeFile].
+ * 4. Call [close] when done.
+ */
+@ExperimentalEnvironmentApi
+interface Environment : AutoCloseable {
+
+  /**
+   * Initialize the environment (e.g. create the working directory).
+   *
+   * The default implementation is a no-op; implementations must be idempotent and safe to call
+   * concurrently.
+   *
+   * @param context A readonly view of the invocation, identifying the calling session, or `null`
+   *   when no context is available (e.g. a `getTools` call that supplies none).
+   */
+  suspend fun initialize(context: ReadonlyContext? = null) {}
+
+  /**
+   * Release resources held by the environment.
+   *
+   * Called when the environment is no longer needed. The default implementation is a no-op;
+   * implementations should ensure this method is idempotent.
+   */
+  override fun close() {}
+
+  /**
+   * Execute a shell command in the working directory.
+   *
+   * Ordinary process outcomes (non-zero exit, timeout) are returned in the [ExecutionResult]; a
+   * failure to launch the command is a [Result.failure] wrapping an [EnvironmentException].
+   *
+   * @param context The tool-call context, identifying the calling session.
+   * @param command The shell command string to execute.
+   * @param timeout Maximum execution time; `null` means no limit.
+   * @return A [Result] wrapping an [ExecutionResult] (exit code, stdout, stderr, timeout status),
+   *   or a [Result.failure] with an [EnvironmentException] if the command could not be launched.
+   */
+  suspend fun execute(
+    context: ToolContext,
+    command: String,
+    timeout: Duration? = null,
+  ): Result<ExecutionResult>
+
+  /**
+   * Read a file from the environment filesystem.
+   *
+   * @param context The tool-call context, identifying the calling session.
+   * @param path Absolute or working-dir-relative path to the file.
+   * @return A [Result] wrapping the raw file contents, or a [Result.failure] with an
+   *   [EnvironmentException] if the file does not exist or cannot be read.
+   */
+  suspend fun readFile(context: ToolContext, path: String): Result<ByteArray>
+
+  /**
+   * Write content to a file in the environment's filesystem.
+   *
+   * Parent directories are created automatically if they do not exist.
+   *
+   * @param context The tool-call context, identifying the calling session.
+   * @param path Absolute or working-dir-relative path to the file.
+   * @param content The raw bytes to write.
+   * @return [Result.success] on success, or a [Result.failure] with an [EnvironmentException] if
+   *   the write fails.
+   */
+  suspend fun writeFile(context: ToolContext, path: String, content: ByteArray): Result<Unit>
+}