Tests: Improve REST API test isolation

[sirreal]

Summary

• Isolates REST authentication flags, block registrations, widget factories, sidebar state, theme features, script globals, and mocked URL fixtures.
• Replaces four external DNS-dependent font-face fixtures with a same-host URL accepted by wp_http_validate_url().
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• Minimized seed 1789071003 to an intermittent rest_invalid_param response caused by fonts.gstatic.com DNS resolution, then passed 20/20 minimized fixed repetitions and replayed the full failing seed.
• 100 consecutive fresh randomized REST API group runs passed on gated HEAD cd85108fe3; each completed 3,550 tests and 16,245 assertions with four known warnings and six skips.
• Exact seeds 1789081001 through 1789081100 produced zero missing ordinals, duplicate ordinals, seed errors, nonzero exits, or summary drift.
• A supplemental full random-order smoke on this exact isolated PR head passed the same 3,550/16,245 baseline. The DNS patch ID and nine changed file trees match the gated head; this PR additionally retains its previously verified deterministic attachment-URL fixture.
• PHP syntax checks and git diff --check passed.

Trac: https://core.trac.wordpress.org/ticket/65893

[github-actions]

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