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/api_client.py

@@ -138,6 +138,8 @@ class APIClient:
                     raise
                 await asyncio.sleep(2 ** attempt)
 
+        raise HomeAssistantError("API request failed after all retries")
+
     async def create(
         self,
         model: str,