Fall back to the browser when a GitHub link cannot be resolved

[copilot-swe-agent]

Description

When githubPullRequests.openPullLinks is enabled, GHPR registers itself as the preferred opener for supported GitHub pull request and issue URLs. If the active GitHub authentication session cannot access the linked repository, GHPR cannot resolve the item but still consumes the link. The user sees an error and the URL never reaches the browser.

This is especially disruptive in the Agents Window, where users may not be able to disable the opener for that window after encountering the failure.

Steps to reproduce

1. Enable githubPullRequests.openPullLinks.
2. Open a pull request or issue URL for a private github.com repository that is not accessible to GHPR's active authentication session.
3. Observe that GHPR reports that it cannot find the item and does not open the URL externally.

Expected behavior

GHPR should open resolvable links in its overview editor. If a pull request or issue cannot be resolved, it should fall back to VS Code's default external opener so the user can continue in the browser.

A fix is proposed in #8958.