#3125 fix(server): terminate streamable HTTP sessions on manager shutdown

Summary

Fixes #2150.

Streamable HTTP sessions were not terminated during manager shutdown, and terminate() did not close active SSE writers. That left EventSourceResponse / ASGI callables incomplete on CTRL+C (uvicorn incomplete-response noise and resource leaks under multi-session hosts).

Changes

1. StreamableHTTPSessionManager.run() — call terminate() on active transports before cancelling the task group.
2. StreamableHTTPServerTransport.terminate() — close all _sse_stream_writers via close_sse_stream() first so SSE responses can finish.
3. Regression tests for writer close + manager shutdown terminate.

Relation to existing work

• Open PR #2253 targets v1.x, is older, and does not include the regression suite. This PR targets main with tests.
• Happy to close/adjust if maintainers prefer landing via #2253 or a different base.

Test plan

• uv run pytest tests/server/test_streamable_http_manager.py::test_terminate_closes_active_sse_stream_writers -q
• uv run pytest tests/server/test_streamable_http_manager.py::test_manager_shutdown_terminates_active_sessions -q
• uv run pytest tests/server/test_streamable_http_manager.py -q (32 passed locally)