Prevent the Active Pull Request view from disappearing

[Copilot]

The Active Pull Request view could disappear when branch resolution selected a newer merged PR or when virtual repositories could not persist PR metadata.

• PR selection

  • Keep explicitly checked-out PRs authoritative for their local branch.
  • Prefer open PRs over newer merged or closed PRs.

• Polling resilience

  • Re-resolve missing metadata for active PRs.
  • Preserve the active view when refresh fails transiently on an unchanged branch.

• Regression coverage

  • Cover explicit checkout, stacked PR selection, and non-persistent metadata scenarios.

• Fixes github pull request icon keep disappearing from the vscode tool bar (left pane) #8830

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.