Add Codespaces checkout option to PR dropdown

[Copilot]

The PR checkout dropdown lacked a direct path to open PRs in Codespaces for testing without local setup.

Changes

• Command: Added pr.checkoutOnCodespacesFromDescription that opens https://github.com/codespaces/new?ref={branch}&repo={owner/repo}
• Menu: Inserted "Checkout on Codespaces" as third option in PR overview dropdown (after "Checkout on vscode.dev")
• Utility: Added codespacesPrLink() helper in utils.ts to construct Codespaces URLs from PR head refs

Implementation

export function codespacesPrLink(pullRequest: { head: GitHubRef }): string {
  const repoFullName = `${pullRequest.head.owner}/${pullRequest.head.name}`;
  const branch = pullRequest.head.ref;
  return `https://github.com/codespaces/new?ref=${encodeURIComponent(branch)}&repo=${encodeURIComponent(repoFullName)}`;
}

The command handler validates PR context and head branch existence before opening the external link, mirroring the pattern used by checkoutOnVscodeDevFromDescription.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add option to checkout PR on Codespaces</issue_title> <issue_description>Testing microsoft/vscode-pull-request-github#7424

It'd be nice if I had an option to checkout the PR on Codespaces. I was working on a local repo that wasn't set up on my machine, and I wish I had the option from the PR header to checkout on Codespaces so that the repo would be set up for me to test my changes

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/vscode-pull-request-github#7450

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