test(grok): configure multipart media mapping

backend/internal/service/openai_gateway_grok_test.go

@@ -749,8 +749,9 @@ func TestForwardGrokMediaImagesEditMultipartConvertsToJSON(t *testing.T) {
 		Type:        AccountTypeAPIKey,
 		Concurrency: 1,
 		Credentials: map[string]any{
-			"api_key":  "api-key",
-			"base_url": "https://xai.test/v1",
+			"api_key":       "api-key",
+			"base_url":      "https://xai.test/v1",
+			"model_mapping": map[string]any{"grok-imagine-edit": "vendor-image-edit"},
 		},
 	}
 	upstream := &httpUpstreamRecorder{resp: &http.Response{