fix(tools): Prevent session drop on MCP tool error

Enable MCP graceful error handling by default and retrieve background session context task exceptions. This prevents unhandled AnyIO TaskGroup transport failures from bubbling through the event loop and abruptly terminating live streaming sessions with 1006 abnormal closure.

Change-Id: Ib4669fb57cddfc6ce61f9a3991b80db3f2b3e9ab