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/protocol/src/models.rs

@@ -1107,7 +1107,10 @@ pub fn local_image_content_items_with_label_number(
             items
         }
         Err(err) => match &err {
-            ImageProcessingError::Read { .. } | ImageProcessingError::Encode { .. } => {
+            ImageProcessingError::Read { .. }
+            | ImageProcessingError::Encode { .. }
+            | ImageProcessingError::InvalidDataUrl { .. }
+            | ImageProcessingError::ImageTooLarge { .. } => {
                 vec![local_image_error_placeholder(path, &err)]
             }
             ImageProcessingError::Decode { .. } if err.is_invalid_image() => {