fix(agent): register explainer config key + shorten footer prefix

Follow-up to the salvaged #34452 turn-completion explainer:
- Register display.turn_completion_explainer: True in DEFAULT_CONFIG so the
  setting is discoverable, matching the file_mutation_verifier precedent.
- Shorten the repeated footer prefix from 'Turn ended without a usable
  reply: ' to 'No reply: ' so the 10 reason variants don't all open with
  the same 8-word boilerplate.
- Update the 7 assertions that referenced the old prefix.

tests/run_agent/test_run_agent.py

@@ -3049,7 +3049,7 @@ class TestRunConversation:
         # #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 "No reply:" in result["final_response"]
         assert result["turn_exit_reason"] == "empty_response_exhausted"
         assert result["api_calls"] == 6  # 1 original + 2 prefill + 3 retries
 
@@ -3072,7 +3072,7 @@ class TestRunConversation:
         assert result["completed"] is True
         # #34452: explanation replaces the bare "(empty)" sentinel.
         assert result["final_response"] != "(empty)"
-        assert "without a usable reply" in result["final_response"]
+        assert "No 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):
@@ -3121,7 +3121,7 @@ class TestRunConversation:
         assert result["completed"] is True
         # #34452: explanation replaces the bare "(empty)" sentinel.
         assert result["final_response"] != "(empty)"
-        assert "without a usable reply" in result["final_response"]
+        assert "No reply:" in result["final_response"]
         assert result["api_calls"] == 4  # 1 original + 3 retries
 
     def test_truly_empty_response_succeeds_on_nudge(self, agent):
@@ -3219,7 +3219,7 @@ class TestRunConversation:
         assert result["completed"] is True
         # #34452: explanation replaces the bare "(empty)" sentinel.
         assert result["final_response"] != "(empty)"
-        assert "without a usable reply" in result["final_response"]
+        assert "No 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."""
@@ -3248,7 +3248,7 @@ class TestRunConversation:
         # #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"]
+        assert "No 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}"

tests/run_agent/test_turn_completion_explainer.py

@@ -159,7 +159,7 @@ def test_run_conversation_empty_exhausted_surfaces_explanation():
     # The user must NOT be left with a bare sentinel; the explanation wins.
     assert result["final_response"] != "(empty)"
     assert result["final_response"].strip() != ""
-    assert "without a usable reply" in result["final_response"]
+    assert "No reply:" in result["final_response"]
 
 
 def test_run_conversation_normal_reply_stays_quiet():
@@ -178,4 +178,4 @@ def test_run_conversation_normal_reply_stays_quiet():
 
     assert result["turn_exit_reason"].startswith("text_response")
     assert result["final_response"] == "Done."
-    assert "without a usable reply" not in result["final_response"]
+    assert "No reply:" not in result["final_response"]