fix: buffer stdio server writes during progress notifications

[2830500285]

Summary

• buffer the server stdio write stream by one message so a handler can queue its final response while stdout is still flushing an earlier notification
• add a regression test covering a slow notification write followed by a response send

Fixes #1141.

Validation

• uv run --frozen --no-sync pytest tests/server/test_stdio.py tests/shared/test_progress_notifications.py -q
• uv run --frozen --no-sync ruff format --check src/mcp/server/stdio.py tests/server/test_stdio.py
• uv run --frozen --no-sync ruff check src/mcp/server/stdio.py tests/server/test_stdio.py
• uv run --frozen --no-sync pyright src/mcp/server/stdio.py tests/server/test_stdio.py

Note: local uv run --frozen sync attempted to fetch the git dependency strict-no-cover, but GitHub fetches for that dependency were failing from this machine. I used the already-synced virtualenv with --no-sync for the targeted checks above.