refactor(langchain): clarify session commit policy helper

[ehz0ah]

Summary

• Rename the LangChain session commit helper from maybe_commit_session to apply_commit_policy.
• Update OpenVikingChatMessageHistory and OpenVikingContextMiddleware to call the renamed helper after message capture.
• Add regression coverage that the new helper is exposed and the legacy helper name is no longer present.

Rationale

The helper does more than opportunistically commit a session: it applies the configured OpenVikingCommitPolicy across never, always, and pending_tokens modes. The new name better matches the behavior used by both the chat history adapter and LangGraph middleware, and keeps the lifecycle API easier to reason about before adding more integration surfaces.

Validation

• uv run --extra dev ruff format openviking/integrations/langchain/client.py openviking/integrations/langchain/history.py openviking/integrations/langchain/middleware.py tests/unit/test_langchain_integration.py
• uv run --extra dev ruff check openviking/integrations/langchain/client.py openviking/integrations/langchain/history.py openviking/integrations/langchain/middleware.py tests/unit/test_langchain_integration.py
• uv run --extra test pytest tests/unit/test_langchain_integration.py --no-cov -q

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