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_request_details_test.go

@@ -1,7 +1,9 @@
 package handlers
 
 import (
+	"net/http"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -116,3 +118,22 @@ func TestGetRequestDetails_PreservesSuffix(t *testing.T) {
 		})
 	}
 }
+
+func TestGetRequestDetails_ImageModelReturns503(t *testing.T) {
+	handler := NewBaseAPIHandlers(&sdkconfig.SDKConfig{}, coreauth.NewManager(nil, nil, nil))
+
+	_, _, errMsg := handler.getRequestDetails("gpt-image-2")
+	if errMsg == nil {
+		t.Fatalf("expected error for gpt-image-2, got nil")
+	}
+	if errMsg.StatusCode != http.StatusServiceUnavailable {
+		t.Fatalf("unexpected status code: got %d want %d", errMsg.StatusCode, http.StatusServiceUnavailable)
+	}
+	if errMsg.Error == nil {
+		t.Fatalf("expected error message, got nil")
+	}
+	msg := errMsg.Error.Error()
+	if !strings.Contains(msg, "/v1/images/generations") || !strings.Contains(msg, "/v1/images/edits") {
+		t.Fatalf("unexpected error message: %q", msg)
+	}
+}