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/GeminiFormFields.tsx

@@ -127,7 +127,7 @@ export function GeminiFormFields({
             id="gemini-model"
             value={model}
             onChange={(e) => onModelChange(e.target.value)}
-            placeholder="gemini-2.5-pro"
+            placeholder="gemini-3-pro-preview"
           />
         </div>
       )}