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()

Bắt đầu với phương thức terminate() trong src/mcp/server/streamable_http.py, sau đó so sánh với _handle_stateless_request() trong src/mcp/server/streamable_http_manager.py. Xác nhận rằng việc dọn dẹp stateless được phân biệt với việc kết thúc phiên và rằng các lần kết thúc phiên thực sự vẫn giữ nguyên hành vi ghi log hiện có.