Fix intermittent test_window_state_change failure on macOS x86-64

[TensorDevLJ]

Fixes #3897.

Summary

Fix an intermittent macOS x86-64 failure in test_window_state_change by waiting for the window size to stabilize before considering a state transition complete.

Also fix the timeout path in WindowProbe.wait_for_window() so it correctly raises on timeout.

This change only affects the Cocoa test probe and does not modify the production window-state implementation.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

[freakboy3742]

Thanks for the PR - that definitely sounds like a plausible analysis; I'll poke the CI task a few times and see if I can reproduce (... or not reproduce) the problem.

On a minor style note: Please direct your AI agent to be more terse. The detail you've provided is helpful in the PR comment; it's less helpful to have a dozen lines of explanation in code.

Similarly, there's no need to describe your testing regimen in the PR comment. We have CI, and a strict coverage requirement. Unless there's extraordinary circumstances, the testing regimen is "pass CI". There's no need to tell us how you ran CI - that's documented as part of the development configuration.

[freakboy3742]

I've re-run the test suite 10 times on macOS-x86-64, and other than a handful of attempts last night where Github decided it didn't want to download files, they've all succeeded - so I'm going to call this successful. Thanks for the PR!