Gracefully fall back to patch apply when bundle push transport fails

[Copilot]

The Changeset Generator was intermittently failing its safe_outputs push path on pull_request when patch_format: bundle hit a bundle-apply error, leaving the .changeset file unpushed and the run red. This change keeps bundle transport as the preferred path but avoids turning bundle-specific apply failures into a hard stop.

• Bundle apply fallback

  • Reuses the existing patch apply flow when bundle fetch/apply fails at runtime.
  • Restores the branch to the pre-bundle tip before attempting the patch fallback.
  • Preserves current behavior when bundle transport succeeds.

• Patch apply reuse

  • Extracts the existing patch-apply/base-commit preparation path into reusable helpers.
  • Keeps patch conflict recovery and failure diagnostics unchanged, now available to bundle fallback as well.

• Regression coverage

  • Adds a focused test for the failure mode where bundle transport exists but cannot be applied, and patch transport succeeds.

Example of the new behavior:

try {
  // preferred path
  await applyBundleTransport();
} catch (bundleError) {
  await exec.exec("git", ["reset", "--hard", remoteHeadBeforePatch], baseGitOpts);
  const patchFallbackResult = await applyPatchTransport();
  if (patchFallbackResult) {
    return patchFallbackResult;
  }
}