image: add shared data URL preparation utilities

[fjord-oai]

Summary

Add shared image-processing primitives needed for centralized image preparation in a follow-up PR.

• Add load_data_url_for_prompt for decoding and preparing base64 image data URLs.
• Add configurable maximum-dimension and 32px patch-budget resizing.
• Enforce a 1 GiB sanity limit on both encoded and decoded data-URL representations.
• Preserve original PNG, JPEG, and WebP bytes when resizing is unnecessary.
• Preserve the existing GIF-to-PNG behavior.
• Move image utility tests into the existing sidecar test module.

Behavior

This PR is intended to be runtime behavior-preserving.

Existing production callers continue using PromptImageMode::ResizeToFit and PromptImageMode::Original with their existing semantics. The new data-URL entrypoint and configurable resize mode have no production callers in this PR; they are used by the next PR in the stack.

This PR does not change user-input handling, view_image, history insertion, request construction, HTTP image URL forwarding, or app-server behavior.

git stack

• 👉 1 image: add shared data URL preparation utilities #27245
• ⏳ 2 core: resize all history images behind a feature flag #27247
• ⏳ 3 core: strip image detail from Responses Lite requests #27246
• ⏳ 4 image: preserve metadata when resizing prompt images #27266

[rka-oai]

Just flagging, responsesapi doesn't throw a direct 400 error for any size (and will just fail later during processing or context construction), but I'm aligned with this max limit, since it would almost certainly fail during processing at this size.