deleteSubAgent doesn't stick while a client is still connected #2003

Steps:

1. A client opens a WebSocket to a sub-agent (a /sub/... path through the parent).
2. The parent calls deleteSubAgent for that sub-agent. hasSubAgent now returns false.
3. The client sends one more message, or just disconnects.
4. hasSubAgent returns true again. The sub-agent is back, with fresh state.

Expected: the sub-agent stays deleted.

Cause: the parent routes every incoming WebSocket frame with _cf_resolveSubAgent, which creates the sub-agent when it doesn't exist. deleteSubAgent doesn't close open connections. So the old connection survives the delete, and its next message (or its close event) recreates the sub-agent it was pointing at.

Repro on main (a68736f), using the existing test agents:

const parent = await getAgentByName(env.TestSubAgentParent, parentName);
await parent.subAgentPing(childName);
const ws = await connectWS(
  `/agents/test-sub-agent-parent/${parentName}/sub/counter-sub-agent/${childName}`
);

await parent.subAgentDelete(childName);
expect(await parent.has("CounterSubAgent", childName)).toBe(false);

ws.send("straggler"); // or just ws.close()
await expect.poll(() => parent.has("CounterSubAgent", childName)).toBe(true);

Workaround: in _cf_forwardSubAgentWebSocketMessage and _cf_forwardSubAgentWebSocketClose, check hasSubAgent first and drop the frame when the sub-agent is gone.