Add typed call responses and validated acknowledgements

[atomiechen]

Summary

• Validate and serialize Pydantic request data passed to call() across sync and async Server/Client APIs.
• Add optional response_model validation for acknowledgements and validate handler return annotations before serializing model responses.
• Preserve native Socket.IO arguments, tuple acknowledgements, Python 3.8 support, and the existing monkey patch path.

Verification

• 12 tests passed locally, including a real client/server round trip in both directions.
• Python 3.8 runtime smoke check passed with the locked dependencies.
• Ruff and Pyright type assertions passed.

Related to #1. Thanks @PythonFZ for the typed response and handler return validation proposal.