fix: emit guardrail halt message to client before closing stream

When the tool loop guardrail fires (max_tool_failures, etc.), the
turn exits with guardrail_halt but no final assistant message was
emitted to the client. The SSE stream closed silently —
indistinguishable from a crash.

The stream_delta_callback(None) before tool execution is a display
flush, not a hard close. After generating the halt response, emit
it through both _safe_print (CLI) and stream_delta_callback (SSE)
so clients see the explanation.

Fixes #30770

agent/conversation_loop.py

@@ -3470,6 +3470,19 @@ def run_conversation(
                         f"⚠️ Tool guardrail halted {decision.tool_name}: {decision.code}"
                     )
                     messages.append({"role": "assistant", "content": final_response})
+                    # Emit the halt message to the client so it's not
+                    # indistinguishable from a crash.  The stream display
+                    # was flushed (callback(None)) before tool execution,
+                    # but the callback is still alive — fire the text
+                    # through it so SSE/TUI clients see the explanation.
+                    if final_response:
+                        agent._safe_print(f"\n{final_response}\n")
+                        if agent.stream_delta_callback:
+                            try:
+                                agent.stream_delta_callback(final_response)
+                                agent.stream_delta_callback(None)
+                            except Exception:
+                                pass
                     break
 
                 # Reset per-turn retry counters after successful tool