Add code-preview-title to name code-preview iframes

[cwickham]

Description

Closes #14770.

The iframe from code-preview has no title, so it has no accessible name (axe-core frame-title). This PR adds a code-preview-title code block attribute. Quarto copies its value to the title of the iframe:

```{.yaml code-preview="example.qmd" code-preview-title="Example presentation with smaller text"}
format:
  revealjs:
    smaller: true
```

Without the attribute, the output does not change.

@cderv: code-preview is internal and has no documentation. Do we want a changelog entry for it? I added one under Accessibility, and I can remove it.

A follow-up quarto-web PR will add titles to the 27 previews on quarto.org.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

AI-assisted PR

• AI tool used: Claude Code
• Codebase grounding: local clone
• Human review: I have reviewed, tested, and verified the AI-generated content before submitting.

[posit-snyk-bot]

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