src/handlers/harness/invoke/invoke.screen.test.tsx flaky test

Example: https://github.com/aws/agentcore-cli/actions/runs/33711478004/job/100511721210?pr=2175

Error:
```
26 | timeoutMs = 1000,
27 | intervalMs = 5,
28 | ): Promise {
29 | let waited = 0;
30 | while (!(await predicate())) {
31 | if (waited >= timeoutMs) throw new WaitForTimeoutError();
^
WaitForTimeoutError: waitFor: condition not met before timeout
at waitFor (/Users/runner/work/agentcore-cli/agentcore-cli/src/testing/timing.tsx:31:36)
at async (/Users/runner/work/agentcore-cli/agentcore-cli/src/handlers/harness/invoke/invoke.screen.test.tsx:441:11)

Error:
at waitFor (/Users/runner/work/agentcore-cli/agentcore-cli/src/testing/timing.tsx:31:36)
at async (/Users/runner/work/agentcore-cli/agentcore-cli/src/handlers/harness/invoke/invoke.screen.test.tsx:441:11)
(fail) invoke chat screen > backspace edits the prompt after a completed send [1384.79ms]

```

Start with src/handlers/harness/invoke/invoke.screen.test.tsx around line 441 and the waitFor implementation in src/testing/timing.tsx. Run the failing invoke chat screen test repeatedly and trace the condition used after sending; done means the backspace-after-completed-send test passes reliably without timeout.

Written by the indexing model from the issue text.