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.toml

@@ -47,6 +47,7 @@ members = [
     "ext/extension-api",
     "ext/goal",
     "ext/guardian",
+    "ext/image-generation",
     "ext/memories",
     "ext/web-search",
     "external-agent-migration",
@@ -165,6 +166,7 @@ codex-execpolicy = { path = "execpolicy" }
 codex-extension-api = { path = "ext/extension-api" }
 codex-goal-extension = { path = "ext/goal" }
 codex-guardian = { path = "ext/guardian" }
+codex-image-generation-extension = { path = "ext/image-generation" }
 codex-external-agent-migration = { path = "external-agent-migration" }
 codex-external-agent-sessions = { path = "external-agent-sessions" }
 codex-experimental-api-macros = { path = "codex-experimental-api-macros" }