[Dashboard] Fix: Handle ping event (#7287)

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This PR adds a simple `ping` case in the `promptNebula` function to avoid `console.warn` logs in the browser console.

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## PR-Codex overview
This PR adds support for a new event type, `"ping"`, in the `ChatStreamedEvent` type definition and handles the `"ping"` event case in the event processing logic.

### Detailed summary
- Added a new case for `"ping"` in the event handling switch statement.
- Introduced a new type definition for the `"ping"` event in the `ChatStreamedEvent` type, including a `data` field of type `string`.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

- **New Features**
  - Improved chat event handling with support for a new "ping" event type, enhancing the stability of real-time chat updates.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: PaoloRollo

apps/dashboard/src/app/nebula-app/(app)/api/chat.ts

@@ -202,6 +202,10 @@ export async function promptNebula(params: {
         break;
       }
 
+      case "ping": {
+        break;
+      }
+
       default: {
         console.warn("unhandled event", event);
       }
@@ -298,4 +302,8 @@ type ChatStreamedEvent =
   | {
       event: "error";
       data: string;
+    }
+  | {
+      event: "ping";
+      data: string;
     };