image: preserve metadata when resizing prompt images - #27266

## 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` #27245
- ⏳ `2` #27247
- ⏳ `3` #27246
- ⏳ `4` #27266

## Summary

Adds complete client-side image preparation behind the default-off
`resize_all_images` feature flag.

When enabled, local image producers defer decoding and resizing. Images
are prepared centrally before insertion into conversation history,
covering user input, `view_image`, and structured tool-output images.

## Behavior

- Processes base64 `data:` images in messages and function/custom tool
outputs.
- Leaves non-data URLs, including HTTP(S) URLs, unchanged.
- Applies image-detail budgets:
  - `high` and omitted: 2048px maximum dimension and 2.5K 32px patches.
  - `original`: 6000px maximum dimension and 10K 32px patches.
  - `auto`: uses the same 2048px / 2.5K-patch budget as high.
  - `low`: unsupported and replaced with an actionable placeholder.
- Preserves original image bytes when no resize or format conversion is
needed.
- Enforces the shared 1 GiB encoded and decoded data-URL sanity limits.
- Replaces only an image that fails preparation, preserving sibling
content and tool-output metadata.
- Uses bounded placeholders distinguishing generic processing failures,
oversized images, and unsupported `low` detail.
- Prepares resumed and forked history before installing it as live
history without modifying persisted rollouts.

## Flag-Off Behavior

When `resize_all_images` is disabled:

- Existing local user-input and `view_image` processing remains
unchanged.
- Existing decoding and error behavior remains unchanged.
- Arbitrary tool-output images are not processed.
- HTTP(S) image URLs continue to be forwarded unchanged.


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

## Summary

- Strip image `detail` fields from every Responses Lite request.
- Apply stripping to message images and function/custom tool-output
images.
- Transform only the formatted request copy without mutating stored
history.
- Preserve image URLs byte-for-byte, including HTTP(S) URLs, without
downloading, validating, or resizing them.
- Preserve all image `detail` fields for non-Responses-Lite models.

## Motivation

Responses Lite does not support image `detail` tags, so Codex must omit
them whenever `model_info.use_responses_lite` is enabled. This transport
requirement is independent of the `resize_all_images` feature.

Stored history retains the original detail values. This keeps
request-specific formatting isolated from conversation state and
preserves the information for local image preparation and
non-Responses-Lite requests.


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