fix(shared): preserve MCPError payload on pickle

[MukundaKatta]

Summary

• make MCPError pickle-safe by reconstructing exceptions from the full ErrorData payload instead of bare Exception args
• preserve the specialized UrlElicitationRequiredError shape during round-trip reconstruction
• add regression tests for pickling both MCPError and UrlElicitationRequiredError

Testing

• python3 -m py_compile src/mcp/shared/exceptions.py tests/shared/test_exceptions.py
• python3 -m pytest tests/shared/test_exceptions.py -k pickle_roundtrip
• PYTHONPATH=src python3 -m pytest tests/shared/test_exceptions.py -k pickle_roundtrip

Local note: pytest is blocked in this desktop environment because the available interpreter bottoms out before the repo's required typing features (ImportError on TypeAlias from Python 3.9).