feat(models): add hardcoded GPT-Image-2 model support in Codex

- Added `GPT-Image-2` as a built-in model to avoid dependency on remote updates for Codex.
- Updated model tier functions (`CodexFree`, `CodexTeam`, etc.) to include built-in models via `WithCodexBuiltins`.
- Introduced new handlers for image generation and edit operations under `OpenAIAPIHandler`.
- Extended tests to validate 503 response for unsupported image model requests.

sdk/cliproxy/service.go

@@ -1410,7 +1410,7 @@ func buildCodexConfigModels(entry *config.CodexKey) []*ModelInfo {
 	if entry == nil {
 		return nil
 	}
-	return buildConfigModels(entry.Models, "openai", "openai")
+	return registry.WithCodexBuiltins(buildConfigModels(entry.Models, "openai", "openai"))
 }
 
 func rewriteModelInfoName(name, oldID, newID string) string {