[Firebase AI] Refactor automated test structure

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Rearrange some of the automated tests, to make the more flakey ones just not run on CI. Mostly around tools, and image generation, since those seem more likely to hit limits.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/24544728621

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[gemini-code-assist]

Code Review

This pull request refactors the test suite initialization in UIHandlerAutomated.cs by categorizing multi-backend tests into basic, editor-only, and complex groups. This allows for more granular control over test execution based on the environment, such as disabling flakey tests in CI. A review comment correctly identified an incomplete comment on line 175 that should be updated to clearly state that the test selection is based on the environment.