Refactor image content to use a shared ImageReference type (#45543)

## What changed

Represent images in `ContentItem` and `FunctionCallOutputContentItem` with `ImageReference::Inline`, flattened to preserve the existing `image_url` wire format. Update image producers and consumers and regenerate app-server schemas and SDK artifacts.

Preserve the Python SDK's `InputImageContentItem` and `InputImageFunctionCallOutputContentItem` class names during generation.

## Testing

Add a regression test for stable Python image class names and adapt existing image tests to the shared representation.

GitOrigin-RevId: c38a780ac3314c2ac2deb3afc1b93b94b6f93fec

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -1759,6 +1759,20 @@
           "type": "object"
         },
         {
+          "anyOf": [
+            {
+              "properties": {
+                "image_url": {
+                  "type": "string"
+                }
+              },
+              "required": [
+                "image_url"
+              ],
+              "title": "ImageUrlFunctionCallOutputContentItem",
+              "type": "object"
+            }
+          ],
           "properties": {
             "detail": {
               "anyOf": [
@@ -1770,9 +1784,6 @@
                 }
               ]
             },
-            "image_url": {
-              "type": "string"
-            },
             "type": {
               "enum": [
                 "input_image"
@@ -1782,7 +1793,6 @@
             }
           },
           "required": [
-            "image_url",
             "type"
           ],
           "title": "InputImageFunctionCallOutputContentItem",