Improve log clarity for "Terminating session: None" in stateless mode

Description

In stateless HTTP mode, every request logs INFO:mcp.server.streamable_http:Terminating session: None. This is correct behavior — stateless mode creates a new transport with mcp_session_id=None per request and terminates it afterward — but the message is confusing to end users who see it and assume their connection is failing or being dropped.

I maintain an MCP server (ha-mcp) that runs in stateless mode, and this log message regularly causes confusion among users who think they aren't connected.

Suggested change

Differentiate the log message for stateless (session-less) terminations:

# Current (streamable_http.py):
logger.info(f"Terminating session: {self.mcp_session_id}")

# Suggested:
if self.mcp_session_id:
    logger.info(f"Terminating session: {self.mcp_session_id}")
else:
    logger.debug("Stateless request completed, cleaning up transport")

This would:

• Downgrade stateless cleanup to DEBUG (routine, not noteworthy)
• Use wording that doesn't alarm users ("completed" vs "Terminating")
• Keep the existing INFO-level log for actual session terminations

References

• Source: streamable_http.py terminate() method
• Stateless transport creation: streamable_http_manager.py _handle_stateless_request()

src/mcp/server/streamable_http.py の terminate() メソッドから始め、次に src/mcp/server/streamable_http_manager.py の _handle_stateless_request() と比較してください。ステートレスなクリーンアップがセッションの終了と区別されていること、および実際のセッション終了では既存のログ動作が維持されることを確認してください。