fix: terminate active streamable http sessions on shutdown

[raashish1601]

Summary

• terminate active stateful streamable HTTP transports before cancelling the session-manager task group during shutdown
• keep the manager's session registry consistent while transports are being closed
• add a regression that keeps one stateful session alive and verifies shutdown awaits transport termination

Fixes #2150.

Testing

• python -m pytest tests/server/test_streamable_http_manager.py -k 'test_run_terminates_active_stateful_sessions_on_shutdown or test_stateful_session_cleanup_on_graceful_exit'
• python -m ruff check src/mcp/server/streamable_http_manager.py tests/server/test_streamable_http_manager.py