Don't show a refresh error when the PR description tab closes during a refresh - #9145

Merging from the PR description tab can show Error refreshing pull request description: Webview is disposed. This happens when the merge closes the tab: with deleteBranchAfterMerge and a postDone default-branch checkout, the checkout closes the tab.

The merge fires a state change, and that starts refreshPanel(). While it waits for resolveModel(...), the checkout closes the tab and the panel is disposed. When the refresh resumes, updateItemNow reads this._panel.visible. VS Code's WebviewPanel getters throw Webview is disposed after dispose, and refreshPanel's catch shows that error as a toast.

Changes (PullRequestOverviewPanel)

• refreshPanel()
  • Returns early if the panel is already disposed. Previously _panel.visible itself threw here, so the call rejected.
  • Re-checks isDisposed after resolveModel and returns quietly.
  • If it fails after the panel is gone, it logs at debug level instead of showing the toast.
• processPendingUpdates()
  • Stops applying queued updates once the panel is disposed.
  • Before, an update queued before the close (for example from createOrShow) would run updateItemNow, hit the same throw, and make the caller's promise fail.

const pullRequest = await this.resolveModel(this._identity);
// The panel can be closed while the pull request is resolved, such as when merging it checks out the default branch.
if (this.isDisposed) {
	return;
}

Tests

New refreshing a closed panel block in pullRequestOverview.test.ts:

• A merge-triggered refresh where the tab closes before the pull request reloads: no error toast.
• refreshPanel() called after the tab is closed: no lookup and no error.
• An update queued before the close: it is dropped, and createOrShow doesn't fail.

To support these, the createPanel fixture now also returns webviewPanel and the default-branch stub.

• Fixes Merging from the PR description tab shows "Error refreshing pull request description: Webview is disposed" #9143