feat: update Gemini default model and remove Google Official preset model

Updated default model from gemini-2.5-pro to gemini-3-pro-preview across:
- Provider presets (PackyCode, Custom)
- Form field placeholders
- Default configurations
- Test cases

Google Official preset now has empty env config, allowing users to choose
their own model or use application defaults, which is more appropriate for
OAuth-based authentication.

Changes:
- geminiProviderPresets.ts: updated model to gemini-3-pro-preview, removed model from Google Official
- GeminiFormFields.tsx: updated placeholder to gemini-3-pro-preview
- GeminiConfigSections.tsx: updated placeholder to gemini-3-pro-preview
- ProviderForm.tsx: updated default config to gemini-3-pro-preview
- gemini_config.rs: updated test examples to gemini-3-pro-preview

src/components/providers/forms/ProviderForm.tsx

@@ -53,7 +53,7 @@ const GEMINI_DEFAULT_CONFIG = JSON.stringify(
     env: {
       GOOGLE_GEMINI_BASE_URL: "",
       GEMINI_API_KEY: "",
-      GEMINI_MODEL: "gemini-2.5-pro",
+      GEMINI_MODEL: "gemini-3-pro-preview",
     },
   },
   null,