cleanup: drop tests/plugin_load_gate.py helper (scaffolding from 820-pluginloading-gate)

[eduralph]

Background

The tests/plugin_load_gate.py helper module was introduced by the
820-pluginloading-gate bundle to give run-verify.sh's C4 red leg a
non-test_ production file to revert. Because the actual change site was the
test_*.py file itself (a test-fixes-test patch), there was no natural
production file — the helper was manufactured to work around that.

The testbed's run-verify.sh now handles test-only patches natively: it detects
when no non-test production file is present and emits
PDCA-UNVERIFIABLE: test-only patch … (exit 77), routing the item to §6
NEEDS-HUMAN rather than a hard gate-fail. docs/INTEGRATION.md documents this
and explicitly names the pluginloading-gate helper as the "never do this"
worked example.

Action

• Remove tests/plugin_load_gate.py.
• Inline hard_load_failure_message(…) directly into
  tests/test_plugin_registration.py (the call site) — the helper is only used
  there.
• Keep tests/test_plugin_load_gate.py unchanged (the regression test for the
  gate behaviour itself).

Net change: one file deleted, one import removed/inlined. No behaviour change.

Source

Act review 2026-06-20 — issue_820-pluginloading-gate §10 #2.