Offer to fork when "Push Commits" targets a read-only remote in Create PR - #9136

Creating a PR on a repo you can't push to (for example, a quick fix in Codespaces) fails when the branch already tracks the read-only remote and has unpushed commits. The Publish Branch path already offers to fork; the Push Commits path pushed straight to the tracked remote. That push either failed, or VS Code's built-in GitHub push-error handling forked behind our back, so the PR head pointed at the original repo instead of the fork.

Changes (CreatePullRequestViewProvider)

• Push through publishBranch: a new pushCommits helper sends Push Commits through FolderRepositoryManager.publishBranch. That gives this path the same permission check, fork prompt, and push-error modals as Publish Branch. If the tracked remote isn't a known GitHub repository, it falls back to a plain push as before.
• Fork becomes the head: when the commits land in a fork, compareOwner/compareRepositoryName are reassigned to the fork. headRepo then resolves to the fork, the head becomes <fork-owner>:<branch>, and the PR still targets the original repo.
• Clean cancel: declining the fork, hitting a rate limit, or a rejected push cancels creation without creating a PR.

const pushResult = await this.pushCommits(compareBranch.upstream.remote, compareBranchName);
if (!pushResult) { /* cancelled */ return; }
if (pushResult.fork) {
	compareOwner = pushResult.fork.owner;
	compareRepositoryName = pushResult.fork.repositoryName;
}

Review notes

• Behaviour change for users with write access: Push Commits now pushes with -u and shows the modal warnings from Publish Branch on failure.
• Model not updated: CreatePullRequestDataModel.compareOwner isn't changed after a fork, which matches the existing publish path. So if creation fails after a fork, a retry from the same view would use the original repo as the head.
• Not covered: creating a patch-n branch automatically when compare == base, as GitHub's web editor does. The view still disables Create in that case.

Tests

New Create pull request with unpushed commits suite in createPRViewProvider.test.ts covers three cases:

• Write access: pushes to origin with -u; head is github:D4.
• Read access, fork accepted: forks, pushes to the fork, and creates the PR with head contributor:D4 against github/test.
• Read access, fork declined: no push and no PR.

• Fixes Auto-fork when when you don't have permissions to commit #2998