feat: exception chain tracking

[kashishhora]

Summary

Added

• New exceptions.py module with capture_exception() for structured error capture
• Stack trace parsing via parse_python_traceback() that walks the traceback chain
• is_in_app() detection to distinguish user code from library code (site-packages, stdlib)
• Exception chain unwrapping for __cause__ (explicit raise from) and __context__ (implicit)
• Context line extraction using linecache for user code frames
• Context variables (store_captured_error/get_captured_error) to preserve exceptions before MCP SDK converts them
• ErrorData, StackFrame, and ChainedErrorData TypedDicts in types.py
• Comprehensive test suite with 30 tests

Modified

• official/monkey_patch.py: Uses capture_exception() for full stack traces, preserves exceptions before MCP SDK conversion
• community/monkey_patch.py: Same error capture improvements

Test Plan

• All 30 exception tests pass (uv run python -m pytest tests/test_exceptions.py)
• Manually test with a FastMCP server that throws exceptions
• Verify error events in MCPcat dashboard show full stack traces and cause chains

[naji247]

Seems a bit odd to me that the default case here is true and not the otherway around. This function works by saying "is it any of these known formats for 3rd party packages?" whereas I would have guessed the opposite logic of "this matches our inapp expectation".

[kashishhora]

Talked about it offline, this is the same behavior as our TypeScript SDK