fix(toolbox-langchain): avoid duplicated parameter descriptions

[WilliamK112]

Fixes googleapis/mcp-toolbox#1327.

LangChain tools currently use the core tool __doc__ as the BaseTool description. The core docstring intentionally appends an Args: section for Python introspection, but LangChain already receives parameter descriptions through args_schema, so the LLM-facing tool description duplicates every parameter description.

This changes the LangChain wrappers to use the core tool's raw _description when available, while preserving a fallback to __doc__ for compatibility with older or mocked tool objects. Regression coverage checks that the LangChain tool description no longer includes Args: while parameter descriptions remain available through args_schema.

Validation:

• pytest tests/test_client.py tests/test_async_client.py tests/test_tools.py tests/test_async_tools.py
• black --check src tests
• isort --check src tests
• MYPYPATH='./src:../toolbox-core/src' mypy --install-types --non-interactive --cache-dir=.mypy_cache/ -p toolbox_langchain

Note: the full pytest tests suite includes E2E coverage that expects a Toolbox service/version environment, so I used the non-E2E LangChain package test set above for this wrapper change.

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