fix(agent): explain abnormal turn endings instead of blank/partial reply

When a turn ends abnormally after substantive tool calls (empty content
after retries, a partial/truncated stream, exhausted retries, or an
iteration/budget limit), the CLI/TUI response area was left blank or
showed only a fragment (e.g. "The") with no consolidated reason. The
internal turn_exit_reason values (empty_response_exhausted,
partial_stream_recovery, etc.) were never surfaced to the user.

Add a turn-completion explainer that mirrors the existing file-mutation
verifier footer: at turn end, map an abnormal turn_exit_reason to a
short, actionable message and either replace the bare "(empty)"
sentinel or append the reason after a partial fragment. Normal
text_response exits (e.g. a terse "Done.") stay quiet.

Gated by display.turn_completion_explainer (default on) with
HERMES_TURN_COMPLETION_EXPLAINER env override, matching the
file-mutation verifier seam.

Closes #34452

tests/run_agent/test_run_agent.py

@@ -3046,7 +3046,11 @@ class TestRunConversation:
 
         mock_compress.assert_not_called()  # no compression triggered
         assert result["completed"] is True
-        assert result["final_response"] == "(empty)"
+        # #34452: the bare "(empty)" sentinel is now replaced by a
+        # user-visible end-of-turn explanation so the failure isn't silent.
+        assert result["final_response"] != "(empty)"
+        assert "without a usable reply" in result["final_response"]
+        assert result["turn_exit_reason"] == "empty_response_exhausted"
         assert result["api_calls"] == 6  # 1 original + 2 prefill + 3 retries
 
     def test_reasoning_only_response_prefill_then_empty(self, agent):
@@ -3066,7 +3070,9 @@ class TestRunConversation:
         ):
             result = agent.run_conversation("answer me")
         assert result["completed"] is True
-        assert result["final_response"] == "(empty)"
+        # #34452: explanation replaces the bare "(empty)" sentinel.
+        assert result["final_response"] != "(empty)"
+        assert "without a usable reply" in result["final_response"]
         assert result["api_calls"] == 6  # 1 original + 2 prefill + 3 retries
 
     def test_reasoning_only_prefill_succeeds_on_continuation(self, agent):
@@ -3113,7 +3119,9 @@ class TestRunConversation:
         ):
             result = agent.run_conversation("answer me")
         assert result["completed"] is True
-        assert result["final_response"] == "(empty)"
+        # #34452: explanation replaces the bare "(empty)" sentinel.
+        assert result["final_response"] != "(empty)"
+        assert "without a usable reply" in result["final_response"]
         assert result["api_calls"] == 4  # 1 original + 3 retries
 
     def test_truly_empty_response_succeeds_on_nudge(self, agent):
@@ -3209,7 +3217,9 @@ class TestRunConversation:
         ):
             result = agent.run_conversation("answer me")
         assert result["completed"] is True
-        assert result["final_response"] == "(empty)"
+        # #34452: explanation replaces the bare "(empty)" sentinel.
+        assert result["final_response"] != "(empty)"
+        assert "without a usable reply" in result["final_response"]
 
     def test_empty_response_emits_status_for_gateway(self, agent):
         """_emit_status is called during empty retries so gateway users see feedback."""
@@ -3235,7 +3245,10 @@ class TestRunConversation:
         ):
             result = agent.run_conversation("answer me")
 
-        assert result["final_response"] == "(empty)"
+        # #34452: explanation replaces the bare "(empty)" sentinel, but the
+        # status emissions during retries are unchanged.
+        assert result["final_response"] != "(empty)"
+        assert "without a usable reply" in result["final_response"]
         # 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}"