Add GitHub API fallback for HEAD branch in getLocalPullRequests

When a PR is created on the GitHub website (not via VS Code UI), the
local branch has no git config metadata (branch.<name>.github-pr-owner-number)
since associateBranchWithPullRequest was never called.

For the current HEAD branch, if it has an upstream but no PR metadata,
query GitHub via getMatchingPullRequestMetadataFromGitHub() and write
the git config metadata via associateBranchWithPullRequest(). This is a
single API call that makes the local PR tree immediately correct.

Agent-Logs-Url: https://github.com/microsoft/vscode-pull-request-github/sessions/001ddef0-6d36-4dcc-96f7-ed3f3d2901e5

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

Author: Copilot

src/github/folderRepositoryManager.ts

@@ -963,7 +963,35 @@ export class FolderRepositoryManager extends Disposable {
 			})));
 		}
 
-		return models.filter(value => value !== undefined) as PullRequestModel[];
+		const result = models.filter(value => value !== undefined) as PullRequestModel[];
+
+		// For the current HEAD branch, if it has an upstream but no PR metadata,
+		// query GitHub to find an associated PR. This handles PRs created on the
+		// GitHub website that haven't been checked out through the extension.
+		const head = this.repository.state.HEAD;
+		if (head?.name && head.upstream && !result.find(pr => pr.localBranchName === head.name)) {
+			try {
+				const metadata = await this.getMatchingPullRequestMetadataFromGitHub(
+					head,
+					head.upstream.remote,
+					undefined,
+					head.upstream.name,
+				);
+				if (metadata) {
+					await PullRequestGitHelper.associateBranchWithPullRequest(
+						this.repository,
+						metadata.model,
+						head.name,
+					);
+					metadata.model.localBranchName = head.name;
+					result.push(metadata.model);
+				}
+			} catch (e) {
+				Logger.debug(`Error checking GitHub for PR on HEAD branch ${head.name}: ${e}`, this.id);
+			}
+		}
+
+		return result;
 	}
 
 	/**