[WIP] Fix push_to_pull_request_branch for HEAD-only bundles

[Copilot]

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---

This section details on the original issue you should resolve

<issue_title>push_to_pull_request_branch cannot apply HEAD-only filtered bundles</issue_title>
<issue_description>## Summary

push_to_pull_request_branch cannot apply a valid HEAD-only git bundle produced by filtered bundle synthesis. create_pull_request.applyBundleToBranch already supports this bundle shape, but the push handler always fetches refs/heads/<branch> and fails when only HEAD is advertised.

Production reproduction

Run: https://github.com/microsoft/vscode-engineering/actions/runs/31064344569

After the checkout-hook workaround, the agent created the requested changes and emitted push_to_pull_request_branch. Authentication succeeded with gh-aw v0.85.0 (no duplicate Authorization error). Safe outputs then failed:

fatal: couldn't find remote ref refs/heads/fix/notebook-celllist-invalid-index-328987-aw-30926877686
Failed to apply bundle: Failed to fetch bundle

Downloaded bundle heads:

4f80191700da9afc6d0b20b9ec6c81fb376f8714 HEAD

The fork branch remained at e226f0c.

Root cause

Filtered bundle synthesis uses a detached temporary worktree and creates the bundle from <base>..HEAD, so the bundle advertises HEAD rather than refs/heads/<target>. create_pull_request.cjs::applyBundleToBranch handles this by inspecting git bundle list-heads and fetching HEAD:<temp-ref>. push_to_pull_request_branch.cjs lacks that fallback and throws immediately when there are no missing prerequisite commits.

Proposed fix

When the initial named-ref fetch fails without prerequisite errors:

1. inspect git bundle list-heads;
2. prefer the single validated refs/heads/* entry if one exists;
3. otherwise accept exactly one <40-hex> HEAD entry and fetch HEAD:<bundle-temp-ref>;
4. retain existing prerequisite recovery and fail closed for ambiguous or invalid bundles;
5. add a unit/integration test for HEAD-only filtered bundles.</issue_description>

• Closes push_to_pull_request_branch cannot apply HEAD-only filtered bundles #50740

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

• Fixes push_to_pull_request_branch cannot apply HEAD-only filtered bundles #50740