Launch image generation by default (#17153)

## Summary
- Promote `image_generation` from under-development to stable
- Enable image generation by default in the feature registry
- Update feature coverage for the new launch-state expectation
- Add the missing image-generation auth fixture field in a tool registry
test

## Testing
- `just fmt`
- `cargo test -p codex-features`
- `cargo test -p codex-tools` currently fails:
`test_full_toolset_specs_for_gpt5_codex_unified_exec_web_search` needs
its expected default tool list updated for `image_generation`

codex-rs/features/src/tests.rs

@@ -160,6 +160,12 @@ fn use_linux_sandbox_bwrap_is_a_removed_feature_key() {
     );
 }
 
+#[test]
+fn image_generation_is_stable_and_enabled_by_default() {
+    assert_eq!(Feature::ImageGeneration.stage(), Stage::Stable);
+    assert_eq!(Feature::ImageGeneration.default_enabled(), true);
+}
+
 #[test]
 fn image_detail_original_is_a_removed_feature_key() {
     assert_eq!(
@@ -168,12 +174,6 @@ fn image_detail_original_is_a_removed_feature_key() {
     );
 }
 
-#[test]
-fn image_generation_is_under_development() {
-    assert_eq!(Feature::ImageGeneration.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::ImageGeneration.default_enabled(), false);
-}
-
 #[test]
 fn tool_call_mcp_elicitation_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::ToolCallMcpElicitation.stage(), Stage::Stable);