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/GeminiConfigSections.tsx

@@ -61,7 +61,7 @@ export const GeminiEnvSection: React.FC<GeminiEnvSectionProps> = ({
         onBlur={onBlur}
         placeholder={`GOOGLE_GEMINI_BASE_URL=https://your-api-endpoint.com/
 GEMINI_API_KEY=sk-your-api-key-here
-GEMINI_MODEL=gemini-2.5-pro`}
+GEMINI_MODEL=gemini-3-pro-preview`}
         rows={6}
         className="w-full px-3 py-2 border border-border-default dark:bg-gray-800 dark:text-gray-100 rounded-lg text-sm font-mono focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 transition-colors resize-y min-h-[8rem]"
         autoComplete="off"