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