[material_ui, cupertino_ui] Migrate dartpad TODOs to `<callout-box>` (#12120)

## Background

[Dartdoc 9.0.7](https://github.com/dart-lang/dartdoc/pull/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: https://github.com/dart-lang/sdk/issues/63782

## Pre-Review Checklist

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