test(mosaic): drop UserButton layer tests covered by the integration suite

[alexcarpenter]

Description

Stacked on #9846.

First step in moving the Mosaic UserButton tests toward "mostly integration". This removes three test files that test internal layers in isolation (useUserButtonController, resolveUserButtonLayout, and the UserButton wrapper with every layer mocked). The connected integration suite already exercises the same behavior through the real popover.

Coverage of src/features/user-button is unchanged without them (375/380 statements, 327/345 branches). The two wrapper behaviors the integration suite did not cover, showing the fallback while Clerk loads and dropping it once nobody is signed in, now live there.

Follow-ups will fold the use cases in the view and model tests into the integration suite and remove the rest.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

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🦋 Changeset detected

Latest commit: a703c0e

The changes in this PR will be included in the next version bump.

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Project	Deployment	Actions	Updated
clerk-js-sandbox	Ready	Preview	Sep 21, 2026 9:14pm UTC
swingset	Ready	Preview	Sep 21, 2026 9:14pm UTC

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