feat(sdk): allow custom FileStore injection

[StatPan]

Summary

• Add an optional file_store parameter to ConversationState.create() and LocalConversation.__init__()
• Preserve the existing persistence_dir behavior when no custom store is provided
• Document the interaction: an injected file_store owns state/EventLog persistence, while persistence_dir still sets observation paths
• Add focused tests using InMemoryFileStore so this PR only covers the injection point, not any database backend

Motivation

Some SDK users run conversations in stateless or ephemeral environments where the local filesystem is not a durable EventLog store. This change lets callers provide their own FileStore implementation without adding any storage backend to the SDK core.

This is intentionally split from #2902: PostgreSQL support can remain external or be proposed separately. The API question here is only whether local SDK conversations can accept a caller-provided FileStore.

Test plan

• uv run ruff format openhands-sdk/openhands/sdk/conversation/state.py openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py tests/sdk/conversation/local/test_custom_file_store.py
• uv run ruff check openhands-sdk/openhands/sdk/conversation/state.py openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py tests/sdk/conversation/local/test_custom_file_store.py
• uv run pytest tests/sdk/conversation/local/test_custom_file_store.py tests/sdk/conversation/test_directories.py tests/sdk/conversation/local/test_conversation_path_types.py -q
• uv run pre-commit run --files openhands-sdk/openhands/sdk/conversation/state.py openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py tests/sdk/conversation/local/test_custom_file_store.py
• uv run python .github/scripts/check_sdk_api_breakage.py

Resolves StatPan#1

[all-hands-bot]

Clean dependency injection pattern with comprehensive tests and clear documentation. Maintains full backward compatibility while enabling custom FileStore implementations.

[StatPan]

Hi @xingyaoww, #3252 splits out only the minimal custom FileStore injection point from #2902. CI is passing and the bot review approved it, but because this is a fork PR the QA Changes workflow is skipped and no human reviewer is currently requested.

Could you take a look or route this to the right maintainer when you have a chance? Thank you.