Add prune fallback for GitHub Git Data API 404s

[Copilot]

The prune Actions job was correctly identifying stale renders, but failed when deleting them via the Git Data API: POST /git/trees returned 404 in GitHub Actions. This change keeps the current batched delete path and adds a targeted fallback for that failure mode.

• Root cause

  • prune_renders.py selected the correct issue-* paths to remove.
  • The failure occurred during the destructive commit step, specifically on Git Data API tree creation.

• Behavior change

  • Preserve the existing fast path:
    • build a tree with sha: null entries
    • create a commit
    • advance renders
  • If a Git Data mutation call fails with the observed 404, fall back to deleting each file through the Contents API instead of failing the workflow.

• Scope

  • Only the prune deletion path changed.
  • The fallback is limited to Git Data 404 mutation errors; other API failures still raise.

• Regression coverage

  • Added a focused test that simulates POST /git/trees -> 404 and verifies the script switches to per-file DELETE /contents/... requests.

try:
    new_tree = request_api("POST", repo_path("/git/trees"), {...})
    ...
except RuntimeError as exc:
    if not _is_git_data_404(exc):
        raise
    delete_via_contents_api(paths_to_delete)