[quick_actions] Adopt code-excerpts for README

[danielleon-cmd]

Replaces the hand-written Dart snippets in quick_actions's README with <?code-excerpt?> pragmas backed by example/lib/readme_excerpts.dart, so they're validated against compilable, analyzed source instead of free-hand text (the original used an untyped callback parameter and print(), which the repo's lint rules disallow). Also drops the package's ci_config.yaml, whose only purpose was opting it out of that validation.

Follows the same pattern as #12641 ([pointer_interceptor] Adopt code-excerpts for README).

Part of #102679.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA. — pending; the CLA bot flagged this as a first-time contribution from this account.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style (bumped to 1.1.1).
• I updated/added any relevant documentation (doc comments with ///). — not applicable, no public API surface changed.
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹. — documentation-only change: the new readme_excerpts.dart is validated by dart analyze/dart format and the update-excerpts check itself; no runtime behavior changed.
• All existing and new tests are passing.

Test plan

• dart run script/tool/bin/flutter_plugin_tools.dart update-excerpts --packages=quick_actions — no diff after regeneration
• dart run script/tool/bin/flutter_plugin_tools.dart validate --packages=quick_actions — no issues
• flutter analyze in example/ — no issues
• dart format --output=none --set-exit-if-changed on the new excerpts file — no changes needed
• flutter test in the package — all tests pass

Note: The Flutter team is currently trialing Gemini Code Assist for GitHub. Comments from gemini-code-assist[bot] aren't authoritative Flutter-team feedback; I'll wait for a human reviewer's guidance on which automated comments (if any) should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩

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