fix: add missing model property to GoogleGeminiBot

api_key and api_base were refactored to @property but model was not
migrated, causing AttributeError: 'GoogleGeminiBot' object has no
attribute 'model' when using any Gemini model.

models/gemini/google_gemini_bot.py

@@ -34,6 +34,13 @@ class GoogleGeminiBot(Bot):
     def api_key(self):
         return conf().get("gemini_api_key")
 
+    @property
+    def model(self):
+        model_name = conf().get("model") or "gemini-pro"
+        if model_name == "gemini":
+            model_name = "gemini-pro"
+        return model_name
+
     @property
     def api_base(self):
         base = conf().get("gemini_api_base", "").strip()