The salvaged unit tests hand-construct completed futures to lock the
_run_on_mcp_loop contract. Add a live-loop test that reproduces the actual
field trigger: an inner asyncio.wait_for expiry stores a real TimeoutError
on a real future scheduled on the MCP loop. Asserts the fixed loop surfaces
it once, promptly -- not spinning to the outer deadline (which both leaked
memory and masked the real error behind the generic wrapper message).
