Media: Require image dimensions for auto-sizes

[Sukhendu2002]

This PR addresses an image-height regression reported in WordPress 6.7.1 when an image uses sizes="auto" without both width and height attributes. The enlarged contain-intrinsic-size: 3000px 1500px workaround added for #62413 can cause incomplete image markup to receive a 1500px intrinsic height. This is visible while lazy-loaded images are waiting to load and can produce significant layout distortion.

Trac ticket: https://core.trac.wordpress.org/ticket/62515

Images with valid dimensions retain the larger intrinsic-size workaround introduced for #62413. Images without complete dimensions instead use the browser’s standard fallback and no longer receive WordPress’s 1500px intrinsic height.

This change does not attempt to infer or reconstruct a missing image aspect ratio. If a theme or plugin removes dimensions after Core generates the markup, the CSS guard prevents WordPress’s enlarged fallback from applying, but the browser must still use its standard fallback.
Author CSS that explicitly overrides the mapped aspect ratio, such as aspect-ratio: auto, is outside the scope of this change.

Use of AI Tools

AI assistance: Yes
Tool(s): Pi
Model(s): GPT-5.6
Used for: Initial code skeleton and test suggestions and code review; final implementation and tests were reviewed and edited by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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