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/src/compact_tests.rs

@@ -332,7 +332,9 @@ fn build_compacted_history_preserves_user_message_passthrough_metadata() {
                 },
                 metadata: Some(CodexHarnessMetadata::default()),
             },
-            ResponseItemEnvelope::new(user_message("summary text")),
+            ResponseItemEnvelope::new(ContextualUserFragment::into(CompactionSummary::new(
+                "summary text",
+            ))),
         ]
     );
 }