…12120)

## Background

[Dartdoc 9.0.7](dart-lang/dartdoc#4264) added support for `<callout-box>` (what we used to call the _blue container_).

## What's changed

This PR migrates all should-be Dartpad instances in API docs to `<callout-box>`. This is what it looks like:

<img width="722" height="640" alt="image" src="https://github.com/user-attachments/assets/c508c8b1-9b97-4500-9aea-cf2bce81df0a" />

This PR also disables `unintended_html_in_doc_comment` because `<callout-box>` is not recognized by the linter for now. An issue has been filed: dart-lang/sdk#63782

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^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.

…12146)

## Background

[Dartdoc 9.0.7](dart-lang/dartdoc#4264) added support for `<callout-box>` (what we used to call the _blue container_).

## What's changed

This PR migrates all former snippet instances in API docs to `<callout-box>`. This is what it looks like:

<img width="734" height="302" alt="image" src="https://github.com/user-attachments/assets/cd0bd77c-4ab2-4875-8fbe-e838e0902c16" />

This PR also disables `unintended_html_in_doc_comment` because `<callout-box>` is not recognized by the linter for now. An issue has been filed: dart-lang/sdk#63782

## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^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.