core: resize all history images behind a feature flag (#27247)

## 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` 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/core/src/prompt_debug.rs

@@ -5,7 +5,6 @@ use codex_exec_server::ExecServerRuntimePaths;
 use codex_login::AuthManager;
 use codex_protocol::error::CodexErr;
 use codex_protocol::error::Result as CodexResult;
-use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::user_input::UserInput;
@@ -78,8 +77,7 @@ pub(crate) async fn build_prompt_input_from_session(
         .await;
 
     if !input.is_empty() {
-        let input_item = ResponseInputItem::from(input);
-        let response_item = ResponseItem::from(input_item);
+        let response_item = sess.response_item_from_user_input(turn_context.as_ref(), input);
         sess.record_conversation_items(turn_context.as_ref(), std::slice::from_ref(&response_item))
             .await;
     }