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