Regression guard for #30770 — verifies the guardrail-halt branch in
agent/conversation_loop.py pushes the synthesized halt message through
stream_delta_callback before breaking out of the loop. Without the
emit, chat-completions SSE writers drain an empty queue and clients
(Open WebUI, etc.) see a finish chunk with zero content delta —
indistinguishable from a crash.
Verified: the test fails when the production fix is reverted.
