Nightly: gem unwind-after-unconverged-supersede e2e fails on bundler 1.17.3/2.1.4 #1685

The nightly CI run on main (39907645034, head aa69125) fails both old-bundler e2e-extended legs:

• e2e-extended (ubuntu-latest, e2e_redirect_gem_build e2e_vendor_gem_build, 3.1, 2.1.4)
• e2e-extended (ubuntu-latest, e2e_redirect_gem_build e2e_vendor_gem_build, 3.1, 1.17.3)

Failing test: gem_hosted_unwind_after_unconverged_supersede_undoes_the_new_block (e2e_redirect_gem_build, 31 passed / 1 failed). It panics at crates/socket-patch-cli/tests/e2e_redirect_gem_build.rs:2346:

rollback: Gemfile:
source "http://127.0.0.1:45027/upstream"
source "http://127.0.0.1:45027/patch-registry/gem/44444444-4444-4444-8444-444444444444/10000000-1a2b-4a1b-8c2d-3e4f5a6b7c8d/" do
  gem "vuln-gem", "1.0.0"
end

The test and its fix both arrived in #1664 (e2b8914, "Fix gem rollback leaving superseding Gemfile block (#1626)"), merged 2026-10-22. PR CI and the merge queue don't run the --ignored gem capstones on the old-bundler legs, so this only shows up in the nightly. e2e_vendor_gem_build passes on the same legs.

This doesn't block merging (ci-ok doesn't include nightly-only legs), but it keeps the nightly on main red. It needs someone to look at the gem unwind path under bundler < 2.2. Either the gen-3 superseding block isn't written in the expected window, or rollback leaves it behind.

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