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/Cargo.lock

@@ -1928,6 +1928,7 @@ dependencies = [
  "codex-git-utils",
  "codex-guardian",
  "codex-hooks",
+ "codex-image-generation-extension",
  "codex-login",
  "codex-mcp",
  "codex-memories-extension",
@@ -3039,6 +3040,31 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "codex-image-generation-extension"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "base64 0.22.1",
+ "codex-api",
+ "codex-core",
+ "codex-extension-api",
+ "codex-features",
+ "codex-login",
+ "codex-model-provider",
+ "codex-model-provider-info",
+ "codex-protocol",
+ "codex-tools",
+ "http 1.4.0",
+ "pretty_assertions",
+ "schemars 0.8.22",
+ "serde",
+ "serde_json",
+ "tempfile",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "codex-install-context"
 version = "0.0.0"