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/api/handlers/handlers.go

@@ -795,6 +795,13 @@ func (h *BaseAPIHandler) getRequestDetails(modelName string) (providers []string
 	parsed := thinking.ParseSuffix(resolvedModelName)
 	baseModel := strings.TrimSpace(parsed.ModelName)
 
+	if strings.EqualFold(baseModel, "gpt-image-2") {
+		return nil, "", &interfaces.ErrorMessage{
+			StatusCode: http.StatusServiceUnavailable,
+			Error:      fmt.Errorf("model %s is only supported on /v1/images/generations and /v1/images/edits", baseModel),
+		}
+	}
+
 	providers = util.GetProviderName(baseModel)
 	// Fallback: if baseModel has no provider but differs from resolvedModelName,
 	// try using the full model name. This handles edge cases where custom models