style: remove unused imports (pass ruff)

- tests/providers/test_anthropic_cache_policy.py: CacheMarkerPolicy was
  imported but never referenced (custom policies in tests use ad-hoc
  duck-typed classes, not the Protocol)
- tests/checkpointer/test_postgres.py: model classes now used in
  test_models_import via __tablename__ assertions for substantive checks

Author: xfgong

tests/checkpointer/test_postgres.py

@@ -15,6 +15,10 @@ def test_models_import() -> None:
     )
     assert EXPECTED_SCHEMA_VERSION == 1
     assert PARTITION_COUNT == 64
+    # All three model classes are reachable via the public model module
+    assert CubepiThread.__tablename__ == "cubepi_threads"
+    assert CubepiMessage.__tablename__ == "cubepi_messages"
+    assert CubepiSchemaVersion.__tablename__ == "cubepi_schema_version"
     # All three tables registered on cubepi_metadata
     assert "cubepi_threads" in cubepi_metadata.tables
     assert "cubepi_messages" in cubepi_metadata.tables

tests/providers/test_anthropic_cache_policy.py

@@ -4,7 +4,6 @@
 
 from cubepi.providers.anthropic import (
     AnthropicProvider,
-    CacheMarkerPolicy,
     DefaultCacheMarkerPolicy,
 )
 from cubepi.providers.base import Message, TextContent, UserMessage