The Browser Preview feature caused GitHub Copilot App to crash unexpectedly.

## Summary

The Browser Preview feature caused GitHub Copilot App to crash unexpectedly.

## Environment

* GitHub Copilot App: 1.0.26
* macOS: 13.7.6
* Hardware: Intel Mac (Macmini8,1)

## What happened

I was using the built-in Browser Preview normally.

During use (or while the Browser Preview was closing), the entire Copilot App crashed immediately.

The application exited without any error dialog.

## Expected behavior

Closing the Browser Preview should cleanly destroy the WebView without crashing the entire application.

## Actual behavior

The application crashes with:

* EXC_BAD_ACCESS (SIGSEGV)
* KERN_INVALID_ADDRESS

The crash occurs on the main thread.

From the crash stack, it appears to happen while Browser Preview is closing a WKWebView:

* `WKWebView::closeAllMediaPresentationsWithCompletionHandler`
* `github_app::browser_preview::close_browser_preview_webview`
* `github_app::browser_preview::close_browser_preview`

This looks like a Browser Preview / WebKit lifecycle issue rather than a model-related problem.

I've attached the complete macOS crash report for investigation.

---
| Field | Value |
| --- | --- |
| App version | 1.0.26 |
| OS | macOS 13.7.6 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 8 |

Start with the attached macOS crash report and the Browser Preview entry points named in the stack: github_app::browser_preview::close_browser_preview_webview and close_browser_preview. Reproduce closing Browser Preview on macOS and trace the WKWebView teardown path. Done means the WebView closes without EXC_BAD_ACCESS or crashing the Copilot App.

索引モデルが issue の本文から書いたものです。