deleteSubAgent doesn't stick while a client is still connected

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:

```ts
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.