image: add shared data URL preparation utilities (#27245)

## 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](https://github.com/magus/git-stack-cli)
- 👉 `1` https://github.com/openai/codex/pull/27245
- ⏳ `2` https://github.com/openai/codex/pull/27247
- ⏳ `3` https://github.com/openai/codex/pull/27246
- ⏳ `4` https://github.com/openai/codex/pull/27266

codex-rs/utils/image/src/error.rs

@@ -25,6 +25,14 @@ pub enum ImageProcessingError {
     },
     #[error("unsupported image `{mime}`")]
     UnsupportedImageFormat { mime: String },
+    #[error("invalid image data URL: {reason}")]
+    InvalidDataUrl { reason: String },
+    #[error("image {representation} is too large ({size} bytes; max {max} bytes)")]
+    ImageTooLarge {
+        representation: &'static str,
+        size: usize,
+        max: usize,
+    },
 }
 
 impl ImageProcessingError {