fix(agents): reject non-positive LangChainAgent max_size

[Bartok9]

Summary

LangChainAgent treated any max_size as valid. With max_size <= 0, the queue is always considered full on the first enqueue path (len >= max_size), so the agent either drops/poplefts incorrectly or never buffers usefully.

Change

• Raise ValueError when max_size <= 0 at construction.
• Offline unit tests with mocked runnable/connectors.

Test plan

• uv run python -m pytest tests/agents/langchain/test_langchain_agent_max_size.py -q

AI-assisted; human-reviewed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.99%. Comparing base (e54f8ca) to head (39a48c3).
⚠️ Report is 16 commits behind head on bartok9/fixes.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                Coverage Diff                @@
##           bartok9/fixes     #815      +/-   ##
=================================================
+ Coverage          73.19%   73.99%   +0.79%     
=================================================
  Files                 82       82              
  Lines               3582     3584       +2     
=================================================
+ Hits                2622     2652      +30     
+ Misses               960      932      -28     

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