Name the failing request when the pull request overview fails to load - #9126

A single transient network error (e.g. UND_ERR_CONNECT_TIMEOUT on GET /user) used to break the PR overview until reload. The error shown, Error updating pull request description: …, also didn't say which request had failed.

The two caching root causes from the issue are already fixed on main. This PR addresses the remaining part: the error message.

Already fixed on main (unchanged here)

• Current user: CredentialStore.ensureCurrentUser makes one request at a time and drops it when it fails, so the next caller retries (Retry current user request after failure #8898, Retry authenticated user lookup after transient failures #8910).
• Repository metadata: GitHubRepository.getMetadata clears _metadata when a request fails, guarded so it can't clear a newer request. Only the deliberate "404 means skip this repository for the session" case stays cached.

Changes

• PullRequestOverviewPanel.updateItemNow: four loads run together (default branch, repository permissions, current user, edit permissions). The existing measure timing helper now also takes a localized label, and a failure is re-thrown with that label:

  measure('currentUser', vscode.l10n.t('Getting the current user'), this._folderRepositoryManager.getCurrentUser(...)),
  // on failure: Error('Getting the current user failed: <cause>')

• Error message: the surfaced message is now Error loading pull request #<n>: <operation> failed: <cause>, and it is also written to the log. Before, it was the misleading Error updating pull request description.
• Regression test (pullRequestOverview.test.ts › "loading errors"):
  • Uses the real FolderRepositoryManager → CredentialStore path, with a cast-free hub whose first GET /user times out.
  • Asserts the error message names the current-user request.
  • Asserts that reopening the overview loads with no new error and makes exactly one more GET /user, which checks the retry end to end.

Notes for review

• In practice the default-branch and permissions loads catch their own errors and fall back. The labels that actually show up are the current-user and edit-permission ones.
• IssueOverviewPanel.updateItem still reports Error updating issue description for a similar combined load. It's left for a follow-up to keep this PR small.

• Fixes Transient connect timeout on GET /user (or repo metadata) permanently breaks PR overview until reload: rejected promises are cached (setCurrentUser / getMetadata) #8899