Tests: Isolate deprecation warning state

[ciampo]

Follow up to #80855.

What?

Isolate deprecation warning state in tests across data, block-editor, components, compose, and interface, and document the patterns in @wordpress/deprecated.

Why?

Deprecation messages are logged once. With shuffle seed 80856, an earlier registry test consumed the registerGenericStore warning before the test that expected it. Several other suites also cleared unrelated deprecation state or left their own messages behind.

How?

The registry suite suppresses its expected deprecation except when asserting the warning. Other suites clear only their own messages before and after tests, preserve warning assertions, and check exact text where applicable. Remove obsolete cleanup from the font-size utility tests and add README examples for both patterns.

Testing Instructions

Run the affected suites with shuffled files and tests. All warning assertions and behavior checks should pass regardless of execution order.

npm run test:unit -- --sequence.shuffle.files --sequence.shuffle.tests --sequence.seed=80856 \
  packages/data/src/test/registry.js \
  packages/interface/src/store/test/actions.ts \
  packages/compose/src/higher-order/pure/test/index.jsdom.test.jsx \
  packages/block-editor/src/components/font-sizes/test/utils.js \
  packages/block-editor/src/components/font-sizes/test/fluid-utils.js \
  packages/block-editor/src/store/test/selectors.jsdom.test.jsx \
  packages/components/src/form-token-field/test/index.jsdom.test.tsx \
  packages/components/src/circular-option-picker/test/index.browser.test.tsx

Testing Instructions for Keyboard

Not applicable; this changes tests and documentation only.

Verification

All 341 affected tests passed with seed 80856, including 11 Chromium tests. Type checking, the full build, documentation generation, and commit hooks passed. JavaScript lint reported no errors and existing assertion warnings. Markdown lint reports three existing issues in the README's autogenerated API section.

Use of AI Tools

Codex helped reproduce the seeded failure, implement and document the cleanup, investigate CI, and run verification.

[github-actions]

🤖 PR meta 🤖

📦 Bundle size

Size Change: 0 B

Total Size: 8.22 MB

_{06aed5a Run}

[github-actions]

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Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Maybe we should add something about unit testing deprecations to the package's README file.

[ciampo]

@Mamaduka hope this looks good!

[Mamaduka]

Looks good 🎉

[ciampo]

I'll actually apply this guidance elsewhere in the repo in this PR, while I'm at it.