Fix search AI preview text contrast in dark mode

[lamek]

Description of what this PR is changing or adding, and why:

Google Programmable Search Engine recently introduced an AI Preview (SGE)
feature. The embedded script injects styles with hardcoded dark gray text
(#4D5156) intended for light backgrounds. In dark mode, this causes the AI
preview summary text, citation snippets, and footer to have very low contrast
against the dark background.

This adds styling overrides in _search.scss for .gsc-sge-* classes
to use the theme's foreground and link color variables.

Before:

After:

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 6e4ec79):

https://flutter-docs-prod--docs-pr13877-fix-search-ai-preview-4y4vved6.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 6e4ec79):

https://flutter-dev-230821--www-pr13877-fix-search-ai-preview-olkdu96l.web.app

[gemini-code-assist]

Code Review

This pull request adds SCSS styles for Google Programmable Search AI Preview (SGE) elements. The feedback suggests grouping selectors with identical style declarations to keep the stylesheet DRY.

[parlough]

Thanks!