Budget retained images during remote compaction (#40280)

## Why

Remote compaction's retained-message budget counted text but not images, so
image-heavy history could retain more context than the budget represented.

## What changed

- Add the opt-in `compaction_image_budget` feature to charge retained images
  using the existing image size estimate.
- Keep images and their adjacent labels atomic when truncating a boundary
  message, while preserving existing text, audio, metadata, and
  client-authored developer-message behavior.
- Stop backfilling older messages when an image at the truncation boundary
  does not fit.

## Testing

- Add unit coverage for image boundaries, annotations, audio, text-only
  messages, and client-authored developer messages.
- Add an integration test covering repeated remote compaction with the feature
  enabled, disabled, and left at its default.

GitOrigin-RevId: 88868bbd09d2858939dbae3f8402e1f497b4e3c6

codex-rs/core/config.schema.json

@@ -640,6 +640,9 @@
             "collaboration_modes": {
               "type": "boolean"
             },
+            "compaction_image_budget": {
+              "type": "boolean"
+            },
             "computer_use": {
               "type": "boolean"
             },
@@ -5610,6 +5613,9 @@
         "collaboration_modes": {
           "type": "boolean"
         },
+        "compaction_image_budget": {
+          "type": "boolean"
+        },
         "computer_use": {
           "type": "boolean"
         },