[url_launcher_ios] Fix test button text to work on iOS 26

[jmagman]

Fixes flutter/flutter#173100

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
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• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request updates the iOS UI tests to handle different button labels for the 'Done' button in SFSafariViewController across different iOS versions. The change correctly uses an NSPredicate to find the button. However, the review identifies two points for improvement: the changelog entry could be clearer for public consumption, and assertions for other UI elements have been removed from the test, which reduces test coverage without explanation. These should be either restored or their removal justified.