Merge origin/main into drop-root-client-server-specific

Resolved merge conflicts keeping TypeAdapter-based validation
(drop RootModel approach) as intended by this branch:
- src/mcp/shared/session.py: Use TypeAdapter adapters
- src/mcp/types.py: Don't import RootModel
- tests/client/test_session.py: Use adapter.validate_python()
- tests/experimental/tasks/client/test_capabilities.py: Use adapter
- tests/test_types.py: Use client_request_adapter

Author: Kludex