fix: prevent already_sent from swallowing empty responses after tool calls (#10531)

When a model (e.g. mimo-v2-pro) streams intermediate text alongside tool
calls ("Let me search for that") but then returns empty after processing
tool results, the stream consumer already_sent flag is True from the
earlier text delivery.  The gateway suppression check
(already_sent=True, failed=False → return None) would swallow the final
response, leaving the user staring at silence after the search.

Two changes:

1. gateway/run.py return path: skip already_sent suppression when the
   final_response is "(empty)" or empty — the user needs to know the
   agent finished even if streaming sent partial content earlier.

2. gateway/run.py response handler: convert the internal "(empty)"
   sentinel to a user-friendly warning instead of delivering the raw
   sentinel string.

Tests added for all empty/None/sentinel cases plus preserved existing
suppression behavior for normal non-empty responses.

gateway/run.py

@@ -3874,6 +3874,18 @@ class GatewayRunner:
                 pass
 
             response = agent_result.get("final_response") or ""
+
+            # Convert the agent's internal "(empty)" sentinel into a
+            # user-friendly message.  "(empty)" means the model failed to
+            # produce visible content after exhausting all retries (nudge,
+            # prefill, empty-retry, fallback).  Sending the raw sentinel
+            # looks like a bug; a short explanation is more helpful.
+            if response == "(empty)":
+                response = (
+                    "⚠️ The model returned no response after processing tool "
+                    "results. This can happen with some models — try again or "
+                    "rephrase your question."
+                )
             agent_messages = agent_result.get("messages", [])
             _response_time = time.time() - _msg_start_time
             _api_calls = agent_result.get("api_calls", 0)
@@ -9400,9 +9412,19 @@ class GatewayRunner:
         # BUT: never suppress delivery when the agent failed — the error
         # message is new content the user hasn't seen, and it must reach
         # them even if streaming had sent earlier partial output.
+        #
+        # Also never suppress when the final response is "(empty)" — this
+        # means the model failed to produce content after tool calls (common
+        # with mimo-v2-pro, GLM-5, etc.).  The stream consumer may have
+        # sent intermediate text ("Let me search for that…") alongside the
+        # tool call, setting already_sent=True, but that text is NOT the
+        # final answer.  Suppressing delivery here leaves the user staring
+        # at silence.  (#10xxx — "agent stops after web search")
         _sc = stream_consumer_holder[0]
         if _sc and isinstance(response, dict) and not response.get("failed"):
-            if (
+            _final = response.get("final_response") or ""
+            _is_empty_sentinel = not _final or _final == "(empty)"
+            if not _is_empty_sentinel and (
                 getattr(_sc, "final_response_sent", False)
                 or getattr(_sc, "already_sent", False)
             ):