Fix checkout of local pull request branches without PR metadata

[Copilot]

This PR fixes an issue where users couldn't check out local pull request branches that weren't originally created through the extension's PR checkout flow.

Problem

The PullRequestGitHelper.checkoutExistingPullRequestBranch() method only looked for branches with PR metadata in git configs. If a user had a local branch with the same name as a PR's head ref but without the associated metadata (e.g., created manually or through other tools), the method would return false instead of attempting to check out the matching local branch.

Solution

Enhanced the method with a fallback mechanism that:

1. Preserves existing behavior: Still prioritizes branches with PR metadata in git configs
2. Adds fallback logic: When no branches with PR metadata are found, attempts to checkout a local branch matching pullRequest.head.ref
3. Auto-associates metadata: When the fallback succeeds, automatically associates the branch with PR metadata for future use
4. Graceful failure: Returns false if no matching local branch exists

Example

Before this fix:

// User has local branch "feature-branch" but no PR metadata
const result = await checkoutExistingPullRequestBranch(repository, pullRequest, progress);
// Returns false, branch not checked out

After this fix:

// User has local branch "feature-branch" matching PR head ref
const result = await checkoutExistingPullRequestBranch(repository, pullRequest, progress);
// Returns true, branch checked out and associated with PR

Testing

Added comprehensive tests covering:

• ✅ Checkout local branch without PR metadata
• ✅ Return false for nonexistent branches
• ✅ Prioritize branches with existing PR metadata
• ✅ Maintain backward compatibility

The implementation follows existing code patterns, includes appropriate error handling and logging, and makes minimal surgical changes to the codebase.

Fixes #6994.

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[alexr00]

@copilot delete the comments

[Copilot]

@copilot delete the comments

Done. Removed the comments from the fallback logic while preserving all functionality. e58b5ec