ThemeProvider: Remove dangerouslySetInnerHTML under FF

[francinelucca]

Relates to https://github.com/github/primer/issues/1049

We believe this is no longer needed now that we've moved away from styled-components, removing under FF to be cautious

Changelog

New

Changed

• Make PreventSSRMismatch a noop under FF

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 4606ec7

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

This PR introduces a feature flag-controlled path in ThemeProvider to skip the SSR “handoff” <script> (and therefore avoid dangerouslySetInnerHTML) while keeping the existing SSR mismatch-prevention behavior available when the flag is off. It also adds coverage and a changeset for the patch release.

Changes:

• Added primer_react_theme_provider_remove_ssr_handoff feature flag and defaulted it to false.
• Updated ThemeProvider to (a) bypass server handoff color mode resolution and (b) skip rendering the SSR handoff script when the flag is enabled.
• Added unit tests asserting the script tag is omitted/present based on the flag value, and added a patch changeset.

Show a summary per file

File	Description
packages/react/src/ThemeProvider.tsx	Gates SSR handoff color mode logic + script rendering behind primer_react_theme_provider_remove_ssr_handoff.
packages/react/src/FeatureFlags/DefaultFeatureFlags.ts	Registers the new feature flag with a default value of false.
packages/react/src/tests/ThemeProvider.test.tsx	Adds tests verifying SSR handoff script rendering behavior under the feature flag.
.changeset/remove-ssr-handoff-script-under-ff.md	Patch changeset documenting the flag-controlled behavior change.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 0

[primer-integration]

Integration test results from github/github-ui PR:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!