telemetry(auth): Enable load telemetry for Toolkit login webview (aws#8100)

## Problem

The Toolkit login webview throws an error when calling `setUiReady()`
because `loadMetadata` is undefined:

```
[error] webviewId="aws.toolkit.AmazonCommonAuth": Error: Webview error
-> Error: Webview backend command failed: "setUiReady()"
-> TypeError: Cannot read properties of undefined (reading 'start')
```

This occurs because `supportsLoadTelemetry` defaults to false in the
base `VueWebview` class, preventing the initialization of `loadMetadata`
during webview setup. Without `loadMetadata`, the webview cannot track
load timing or emit load telemetry.

## Solution

Added one line to enable load telemetry in `ToolkitLoginWebview`.

This flag was originally introduced in
[e7b7307](aws@e7b7307)
to track Amazon Q webview load times and failures. We're now extending
this functionality to the Toolkit login webview for consistent telemetry
across both implementations.




### Before fix:
<img width="2108" height="1109" alt="before-toolkit-auth-webview-error"
src="https://github.com/user-attachments/assets/e53fb12b-6966-45ca-bee5-645e48a09cd4"
/>




Toolkit login webview error showing loadMetadata undefined. This occurs
because `supportsLoadTelemetry` defaults to false in the base
`VueWebview` class, preventing the initialization of `loadMetadata`
during webview setup. Without `loadMetadata`, the webview cannot track
load timing or emit load telemetry. The error happens
[here](https://github.com/aws/aws-toolkit-vscode/blob/da8fc990bc50689e043ff1d7cbde3615d2d7571e/packages/core/src/webviews/main.ts#L401)
when trying to calculate duration:

```typescript
const duration = globals.clock.Date.now() - this.loadMetadata!.start
```

### After fix:

Toolkit login webview successfully loading with duration metrics.

<img width="2107" height="1115"
alt="after-toolkit-auth-webview-with-load-duration"
src="https://github.com/user-attachments/assets/0f9d5bf4-dc5b-49de-a76e-776670ef683d"
/>

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: keenwilson

packages/core/src/login/webview/vue/toolkit/backend_toolkit.ts

@@ -28,6 +28,7 @@ import globals from '../../../../shared/extensionGlobals'
 export class ToolkitLoginWebview extends CommonAuthWebview {
     public override id: string = 'aws.toolkit.AmazonCommonAuth'
     public static sourcePath: string = 'vue/src/login/webview/vue/toolkit/index.js'
+    public override supportsLoadTelemetry: boolean = true
     private isCodeCatalystLogin = false
 
     override onActiveConnectionModified: vscode.EventEmitter<void> = new vscode.EventEmitter()