The two TestSourceGuardrail tests asserted the presence of literal
strings ("#58753", "_user_survives") in context_compressor.py. Those
are change-detector tests that break on any refactor without catching a
real regression. The four behavioral tests in
TestCompressAlwaysKeepsAUserTurn already exercise the real compress()
path and fully cover the invariant (user turn survives, summary pinned
to user, no consecutive user roles, surviving tail user untouched).
Regression coverage for the kanban-worker crash where compression left a
transcript with no user-role messages, triggering a non-retryable
`400 No user query found in messages` from vLLM/Qwen.
Exercises the real `compress()` path with the reporter's shape (no system
prompt in the list, a re-compaction with the only user turn in the
compressed middle) and asserts the output always keeps >=1 user turn,
never introduces consecutive user roles, and leaves a surviving tail user
message untouched. A source guardrail pins the guard so a future refactor
cannot silently drop it.
