Update NativeWebview title only if the title changes (sourcegraph#5861)

`cody-agent-trace.json` looks like this now:
```
[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}


[Trace - 01:03:47 PM] Received notification 'webview/setTitle'
Params: {
  "handle": "native-webview-panel-df3675ed-b3d7-4051-94ff-78a569945443",
  "title": "Explain what @src/main/ja..."
}
```

I think there is no need to send the notification if the title does not
change actually.

## Test plan
1. Run IDE. 
2. Run Explain a few times. 
3. Review `cody-agent-trace.json`
The number of notifications should be reasonable. 

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## Changelog

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agent/src/NativeWebview.ts

@@ -351,10 +351,11 @@ class NativeWebviewPanel implements vscode.WebviewPanel {
     }
 
     public set title(value: string) {
-        this.delegate.setTitle(this.handle, value)
-        this._title = value
+        if (value !== this._title) {
+            this.delegate.setTitle(this.handle, value)
+            this._title = value
+        }
     }
-
     public get iconPath(): vscode.Uri | { light: vscode.Uri; dark: vscode.Uri } | undefined {
         if (!this._iconPath) {
             return undefined