Add feature-gated standalone image generation extension (#24723)

## Why

Add a standalone image generation path that can be exercised
independently of hosted Responses image generation, while retaining the
hosted tool as fallback unless the extension is actually available to
the model.

## What changed

- Added the `codex-image-generation-extension` crate with standalone
generate/edit execution, prior-image selection for edits, model-visible
image output, and local generated-image persistence.
- Installed the extension in app-server behind the disabled-by-default
`imagegenext` feature and backend eligibility checks.
- Updated core tool planning so eligible `image_gen.imagegen` exposure
replaces hosted `image_generation`, while unavailable configurations
retain hosted fallback.
- Added coverage for extension behavior, edit history reuse, feature
gating, auth eligibility, and hosted-tool replacement.
- The extension is installed through app-server only in this PR; other
execution paths retain hosted image generation because hosted
replacement occurs only when the standalone executor is actually
registered and model-visible.
- The initial extension contract intentionally fixes the image model to
`gpt-image-2` and uses automatic image parameters.
- Native generated-image history/card parity and rollout persistence
cleanup are intentionally deferred follow-up work.

## Validation

- `just test -p codex-image-generation-extension`
- `just test -p codex-features`
- `just test -p codex-core
hosted_tools_follow_provider_auth_model_and_config_gates`
- `just test -p codex-app-server`
- `just fix -p codex-image-generation-extension -p codex-features -p
codex-core -p codex-app-server`
- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`

---------

Co-authored-by: jif-oai <jif@openai.com>

codex-rs/core/config.schema.json

@@ -473,6 +473,9 @@
             "image_generation": {
               "type": "boolean"
             },
+            "imagegenext": {
+              "type": "boolean"
+            },
             "in_app_browser": {
               "type": "boolean"
             },
@@ -4564,6 +4567,9 @@
         "image_generation": {
           "type": "boolean"
         },
+        "imagegenext": {
+          "type": "boolean"
+        },
         "in_app_browser": {
           "type": "boolean"
         },