fix(mcp): McpServerController.start/stop race creates a second socket host and leaks the first #1563

Problem

McpServerController.start() (lib/core/mcp/mcp_server_controller.dart:102-124) checks _host != null first, then awaits _version() and host.start(), and assigns _host only after that. Two overlapping calls (startIfEnabled + setEnabled(true), repeated toggle, regenerateToken) create two McpSocketHosts: the first is leaked (never stopped), both write the same endpoint file and the token in the file may not belong to the host that is tracked.

stop() during an in-flight start() sees _host == null, does nothing, and the server comes up afterwards while the setting says disabled.

host.changes.listen is also never cancelled.

Expected

• Serialize start / stop (single in-flight future or a lock) and assign _host before the first await.
• Cancel the changes subscription on stop.
• Test: concurrent start() x2 yields one host; stop() during start() ends stopped.