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()

从 src/mcp/server/streamable_http.py 中的 terminate() 方法开始，然后将其与 src/mcp/server/streamable_http_manager.py 中的 _handle_stateless_request() 进行比较。确认无状态清理与会话终止有所区别，并确认实际的会话终止保留现有的日志行为。