refactor(google-gemini): rewrite integration using google-genai 1.16.0

Completely rewrote the Google Gemini integration logic based on google-genai 1.16.0 to fix issue #6.
Key changes:
- Updated to the latest google-genai library
- Made API endpoint abstract while retaining option for custom endpoint configuration
- Refactored logic and classes exclusively within Google Gemini implementation
- All changes are limited to Google Gemini integration refactoring with no impact on other functionality.

custom_components/ha_text_ai/manifest.json

@@ -13,17 +13,17 @@
   "loggers": ["custom_components.ha_text_ai"],
   "mqtt": [],
   "quality_scale": "silver",
-    "requirements": [
-      "openai>=1.12.0",
-      "anthropic>=0.8.0",
-      "google-generativeai>=0.3.0",
-      "aiohttp>=3.8.0",
-      "async-timeout>=4.0.0",
-      "certifi>=2024.2.2"
-    ],
+  "requirements": [
+    "openai>=1.12.0",
+    "anthropic>=0.8.0",
+    "google-genai>=1.16.0",
+    "aiohttp>=3.8.0",
+    "async-timeout>=4.0.0",
+    "certifi>=2024.2.2"
+  ],
   "single_config_entry": false,
   "ssdp": [],
   "usb": [],
-  "version": "2.1.6",
+  "version": "2.1.7",
   "zeroconf": []
 }