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

[Hweinstock]

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

Error:

26 |   timeoutMs = 1000,
  27 |   intervalMs = 5,
  28 | ): Promise<void> {
  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 <anonymous> (/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 <anonymous> (/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]