Avoid pickle in LangGraph rollout logging

[Kovbo]

A better solution for this issue.

Summary

Removes pickle serialization/deserialization from the LangGraph rollout logger. Structured LangGraph log entries now stay in memory for the duration of the wrapped rollout, while the existing readable text log is still written to disk.

Changes

• Store structured FileLogger entries in memory instead of appending them to a .pkl file.
• Pass the active FileLogger instance through wrap_rollout() into create_messages_from_logs() so LangChain objects are preserved without deserialization.
• Add focused tests covering non-JSON-serializable log entries and the wrapper log-reader contract.

Test plan

• uv run pytest tests/unit/test_langgraph_logging.py
• uv run ruff check src/art/langgraph/logging.py src/art/langgraph/llm_wrapper.py tests/unit/test_langgraph_logging.py
• git diff --check