Extract rule: template-require-iframe-src-attribute

[NullVoxPopuli]

Split from #2371.

[NullVoxPopuli-ai-agent]

Review: template-require-iframe-src-attribute

General Correctness

1. Faithful port of the original. The logic is straightforward: check if <iframe> has a src attribute, report if missing. This matches the original exactly.

2. Original supports auto-fix (adding src="about:blank"), but the PR sets fixable: null. This is acceptable for initial migration; auto-fix can be added later.

3. The error message matches the original exactly. Good.

4. Tests cover the key cases: iframe with src, iframe without src, iframe with other attributes but no src. Both gjs and hbs modes tested.

5. Minor: <iframe src=""></iframe> is treated as valid (has a src attribute, even though empty). The original also allows this since it just checks for attribute presence. Consistent.

Scope Analysis (gjs/gts)

This rule only checks GlimmerElementNode for node.tag === 'iframe' -- a plain HTML element. No scope analysis needed.

Summary

Clean, correct migration. No issues found.

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