REST API: Add tests for global styles endpoint integer ID type

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/61911

This PR adds unit tests for the global styles endpoint ID type fix.

The tests verify that:

1. The endpoint correctly treats IDs as integers
2. The schema properly defines the ID as an integer type
3. The route is registered with the correct ID type and without unnecessary sanitization
4. Updates QUnit fixtures to reflect the endpoint changes

Follow-up of PR #7234.

[github-actions]

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[himanshupathak95]

Hi @johnbillion. I have added the unit tests and updated the existing test files. Now, all the tests are passing. Let me know if the changes look good to you. 🙌🏻