fix(streamable-http): reduce stateless termination log noise

[BlocksecPHD]

Summary

This PR adjusts termination logging in StreamableHTTPServerTransport for stateless mode.

• Keep INFO log for stateful session termination (session id exists)
• Emit DEBUG log for stateless transport cleanup (mcp_session_id is None)
• Update wording to reflect normal request completion in stateless mode

This addresses confusion reported in #2329 where users interpret Terminating session: None as an error condition.

Changes

• src/mcp/server/streamable_http.py
  • Conditional log level/message in terminate()
• tests/server/test_streamable_http_manager.py
  • Add regression test to assert:
    • debug message exists for stateless cleanup
    • info-style Terminating session: None message is not emitted

Validation

Ran targeted tests locally:

python -m pytest tests/server/test_streamable_http_manager.py \
  -k 'stateless_requests_memory_cleanup or stateless_termination_logs_debug_not_info'

Result: 2 passed

[km-anthropic]

@claude review

[dgenio]

The change is focused and correct. A few observations:

Code change: The conditional log level is the right approach — DEBUG for routine cleanup, INFO only for named session termination. The wording "Stateless request completed, cleaning up transport" is much clearer than "Terminating session: None".

Test: Well-structured and mirrors the existing test_stateless_requests_memory_cleanup pattern in the file. Using caplog.at_level(logging.DEBUG, ...) to capture the debug message is correct.

One minor nit: the del message in mock_send is a bit unusual — pass or ... is more idiomatic for a stub that just suppresses lint. Non-blocking though.

For context: #1696 / #1697 is a docs PR covering stateless mode semantics more broadly — this fix and that documentation would complement each other well for users trying to understand what stateless mode does and doesn't do.