Fix: Update default Gemini model in Helm values to gemini-2.0-flash-lite for compatibility (#1001)

The current default model `gemini-1.5-pro` in the Helm `values.yaml`
file causes an error during deployment:

```
Model models/gemini-1.5-pro failed: 404 models/gemini-1.5-pro-002 is not found for API version v1beta, or is not supported for generateContent.
```
Issue which i faced:

<img width="1438" height="858" alt="Screenshot 2025-10-05 at 8 14 12 PM"
src="https://github.com/user-attachments/assets/1a3b64e4-2a76-4ae4-8853-47c0774d33a9"
/>

To resolve this, the default model value has been updated to
`gemini-2.0-flash-lite`, which is available and works correctly with the
`generateContent` API.

**Changes made:**
* Updated the default value for `providers.openAI.model` (or equivalent
key) in `values.yaml` to `gemini-2.0-flash-lite`.

<img width="1426" height="857" alt="Screenshot 2025-10-05 at 7 50 04 PM"
src="https://github.com/user-attachments/assets/1b0db16a-19e6-432f-9039-9b7d1ea5defb"
/>

Signed-off-by: Achanandhi-M <[email protected]>

Author: Achanandhi-M

helm/kagent/values.yaml

@@ -173,7 +173,7 @@ providers:
       azureEndpoint: ""
   gemini:
     provider: Gemini
-    model: "gemini-1.5-pro"
+    model: "gemini-2.0-flash-lite"
     apiKeySecretRef: kagent-gemini
     apiKeySecretKey: GOOGLE_API_KEY
     # apiKey: ""