fix: activate fallback provider on repeated empty responses + user-visible status (#7505)

When models return empty responses (no content, no tool calls, no
reasoning), Hermes previously retried 3 times silently then fell through
to '(empty)' — without ever trying the fallback provider chain. Users on
GLM-4.5-Air and similar models experienced what appeared to be a
complete hang, especially in gateway (Telegram/Discord) contexts where
the silent retries produced zero feedback.

Changes:
- After exhausting 3 empty retries, attempt _try_activate_fallback()
  before giving up with '(empty)'. If fallback succeeds, reset retry
  counter and continue the conversation loop with the new provider.
- Replace all _vprint() calls in recovery paths with _emit_status(),
  which surfaces messages through both CLI (_vprint with force=True)
  and gateway (status_callback -> adapter.send). Users now see:
  * '⚠️ Empty response from model — retrying (N/3)' during retries
  * '⚠️ Model returning empty responses — switching to fallback...'
  * '↻ Switched to fallback: <model> (<provider>)' on success
  * '❌ Model returned no content after all retries [and fallback]'
- Add logger.warning() throughout empty response paths for log file
  visibility (model name, provider, retry counts).
- Upgrade _last_content_with_tools fallback from logger.debug to
  logger.info + _emit_status so recovery is visible.
- Upgrade thinking-only prefill continuation to use _emit_status.

Tests:
- test_empty_response_triggers_fallback_provider: verifies fallback
  activation after 3 empty retries produces content from fallback model
- test_empty_response_fallback_also_empty_returns_empty: verifies
  graceful degradation when fallback also returns empty
- test_empty_response_emits_status_for_gateway: verifies _emit_status
  is called during retries so gateway users see feedback

Addresses #7180.

tests/run_agent/test_run_agent.py

@@ -1823,6 +1823,111 @@ class TestRunConversation:
         assert result["final_response"] == "Here is the actual answer."
         assert result["api_calls"] == 2  # 1 original + 1 nudge retry
 
+    def test_empty_response_triggers_fallback_provider(self, agent):
+        """After 3 empty retries, fallback provider is activated and produces content."""
+        self._setup_agent(agent)
+        agent.base_url = "http://127.0.0.1:1234/v1"
+        # Configure a fallback chain
+        agent._fallback_chain = [{"provider": "openrouter", "model": "anthropic/claude-sonnet-4"}]
+        agent._fallback_index = 0
+        agent._fallback_activated = False
+
+        empty_resp = _mock_response(content=None, finish_reason="stop")
+        content_resp = _mock_response(content="Fallback answer.", finish_reason="stop")
+        # 4 empty (1 orig + 3 retries), then fallback model answers
+        agent.client.chat.completions.create.side_effect = [
+            empty_resp, empty_resp, empty_resp, empty_resp, content_resp,
+        ]
+
+        fallback_called = {"called": False}
+
+        def _mock_fallback():
+            fallback_called["called"] = True
+            # Simulate what _try_activate_fallback does: just advance the
+            # index and set the flag (the client is already mocked).
+            agent._fallback_index = 1
+            agent._fallback_activated = True
+            agent.model = "anthropic/claude-sonnet-4"
+            agent.provider = "openrouter"
+            return True
+
+        with (
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+            patch.object(agent, "_try_activate_fallback", side_effect=_mock_fallback),
+        ):
+            result = agent.run_conversation("answer me")
+        assert fallback_called["called"], "Fallback should have been triggered"
+        assert result["completed"] is True
+        assert result["final_response"] == "Fallback answer."
+
+    def test_empty_response_fallback_also_empty_returns_empty(self, agent):
+        """If fallback also returns empty, final response is (empty)."""
+        self._setup_agent(agent)
+        agent.base_url = "http://127.0.0.1:1234/v1"
+        agent._fallback_chain = [{"provider": "openrouter", "model": "anthropic/claude-sonnet-4"}]
+        agent._fallback_index = 0
+        agent._fallback_activated = False
+
+        empty_resp = _mock_response(content=None, finish_reason="stop")
+        # 4 empty from primary (1 + 3 retries), fallback activated,
+        # then 4 more empty from fallback (1 + 3 retries), no more fallbacks
+        agent.client.chat.completions.create.side_effect = [
+            empty_resp, empty_resp, empty_resp, empty_resp,  # primary exhausted
+            empty_resp, empty_resp, empty_resp, empty_resp,  # fallback exhausted
+        ]
+
+        def _mock_fallback():
+            if agent._fallback_index >= len(agent._fallback_chain):
+                return False
+            agent._fallback_index += 1
+            agent._fallback_activated = True
+            agent.model = "anthropic/claude-sonnet-4"
+            agent.provider = "openrouter"
+            return True
+
+        with (
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+            patch.object(agent, "_try_activate_fallback", side_effect=_mock_fallback),
+        ):
+            result = agent.run_conversation("answer me")
+        assert result["completed"] is True
+        assert result["final_response"] == "(empty)"
+
+    def test_empty_response_emits_status_for_gateway(self, agent):
+        """_emit_status is called during empty retries so gateway users see feedback."""
+        self._setup_agent(agent)
+        agent.base_url = "http://127.0.0.1:1234/v1"
+
+        empty_resp = _mock_response(content=None, finish_reason="stop")
+        # 4 empty: 1 original + 3 retries, all empty, no fallback
+        agent.client.chat.completions.create.side_effect = [
+            empty_resp, empty_resp, empty_resp, empty_resp,
+        ]
+
+        status_messages = []
+
+        def _capture_status(msg):
+            status_messages.append(msg)
+
+        with (
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+            patch.object(agent, "_emit_status", side_effect=_capture_status),
+        ):
+            result = agent.run_conversation("answer me")
+
+        assert result["final_response"] == "(empty)"
+        # Should have emitted retry statuses (3 retries) + final failure
+        retry_msgs = [m for m in status_messages if "retrying" in m.lower()]
+        assert len(retry_msgs) == 3, f"Expected 3 retry status messages, got {len(retry_msgs)}: {status_messages}"
+        failure_msgs = [m for m in status_messages if "no content" in m.lower() or "no fallback" in m.lower()]
+        assert len(failure_msgs) >= 1, f"Expected at least 1 failure status, got: {status_messages}"
+
     def test_nous_401_refreshes_after_remint_and_retries(self, agent):
         self._setup_agent(agent)
         agent.provider = "nous"