fix: suppress completed request cancellation leak

[MukundaKatta]

Summary

• return the underlying CancelScope.exit suppression result from RequestResponder.exit
• swallow a late cancellation from an already-cancelled/completed responder so cancelled requests do not tear down sibling tasks
• add regressions for completed responder cancellations

Fixes #2610.

Tests

• uv run pytest tests/shared/test_session.py -q -k "request_responder or request_cancellation"
• uv run pytest tests/server/test_cancel_handling.py tests/shared/test_session.py -q
• uv run ruff check src/mcp/shared/session.py tests/shared/test_session.py
• uv run ruff format --check src/mcp/shared/session.py tests/shared/test_session.py
• uv run pyright src/mcp/shared/session.py tests/shared/test_session.py