fix: Address code review findings for v2.4.0

- Fix 404 accepted as valid API response in config_flow validation (both ConfigFlow and OptionsFlow)
- Sanitize catch-all exception in config_flow (str(e) → "unknown" error key)
- Add exception chaining (from err) to ConfigEntryNotReady raise
- Replace datetime.now() with dt_util.utcnow() for HA timezone convention
- Add defensive raise after retry loop in api_client._make_request
- Remove unused CONF_API_KEY/CONF_NAME imports from const.py
- Restore google-genai dependency in manifest (required for Gemini provider)

custom_components/ha_text_ai/manifest.json

@@ -12,7 +12,8 @@
   "loggers": ["custom_components.ha_text_ai"],
   "requirements": [
     "aiofiles>=23.0.0",
-    "aiohttp>=3.8.0"
+    "aiohttp>=3.8.0",
+    "google-genai>=1.16.0"
   ],
   "single_config_entry": false,
   "version": "2.4.0"