No longer exiting the application when the RegularWindow is destroyed + resolving the window created promise after runWidget

[mattkae]

This potentially fixes #177715 by:

1. Relying on the test framework to close the app
2. Completing the windowCreated future following runWidget when everything is actually ready to go

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request refactors the windowing integration test. It removes a custom _MainRegularWindowControllerDelegate that explicitly exited the application upon window destruction, now relying on the test framework for application lifecycle management. Additionally, the windowCreated future is now completed after runWidget is called, which ensures the UI is initialized before the test driver interacts with the application.