revert(agent): drop the no-op side-channel length-stop change

The reasoning half only touched a docstring, and its tests pinned main's
existing continuation (reasoning-off retry, then the 'No visible answer'
ceiling). Nothing changed, so the PR stays on the Windows close/stop fix.

agent/turn_truncation.py

@@ -246,9 +246,7 @@ def _abort_reason(agent: Any, content: Any, has_tool_calls: bool) -> Optional[tu
     """``(vprint, user response, error)`` when continuation must NOT be attempted:
     thinking exhausted the budget (reasoning blocks with no visible text after them —
     ``content=None`` from non-<think> models is normal truncation), or a repetition loop
-    burned the budget on one fragment (reasoning stripped first). A length stop whose
-    only text is a reasoning side channel is unfinished thought: continuation owns it.
-    """
+    burned the budget on one fragment (reasoning stripped first)."""
     if has_tool_calls:
         return None
     if content and _THINK_TAG_RE.search(content) and not agent._has_content_after_think_block(content):

tests/agent/test_length_continuation_thinking_exhaustion.py

@@ -142,27 +142,6 @@ def _thinking_only_length_response():
     )
 
 
-def _side_channel_length_response(*, reasoning=None, reasoning_content=None, content=""):
-    """finish_reason=length, empty visible content, reasoning parked off to
-    the side. Model id is irrelevant — the field is the signal."""
-    from tests.agent.test_run_agent import _mock_assistant_msg
-
-    return SimpleNamespace(
-        id="chatcmpl-reasoning-side-channel",
-        model="test/model",
-        choices=[SimpleNamespace(
-            index=0,
-            message=_mock_assistant_msg(
-                content=content,
-                reasoning=reasoning,
-                reasoning_content=reasoning_content,
-            ),
-            finish_reason=FINISH_REASON_LENGTH,
-        )],
-        usage=None,
-    )
-
-
 def _full_response(content):
     from tests.agent.test_run_agent import _mock_response
 
@@ -341,36 +320,3 @@ class TestReasoningOffReachesTheWire:
         calls = loop_agent.client.chat.completions.create.call_args_list
         first = (calls[0].kwargs.get("extra_body") or {}).get("reasoning")
         assert first == {"enabled": True, "effort": "high"}, first
-
-
-class TestReasoningSideChannelLengthContinues:
-    """finish_reason=length with nothing visible and a reasoning side channel is
-    unfinished thought. Continuation owns it; the side channel is not the answer.
-    """
-
-    @pytest.mark.parametrize("field", ["reasoning", "reasoning_content"])
-    def test_empty_visible_content_continues(self, loop_agent, field):
-        kwargs = {field: "the model spent the whole budget here"}
-        loop_agent.client.chat.completions.create.side_effect = [
-            _side_channel_length_response(**kwargs),
-            _full_response("the visible answer"),
-        ]
-        result = _run(loop_agent, "write me a long report")
-
-        assert result["completed"] is True
-        assert "the visible answer" in (result["final_response"] or "")
-        assert "the model spent the whole budget here" not in (result["final_response"] or "")
-        assert loop_agent.client.chat.completions.create.call_count == 2
-
-    def test_visible_answer_with_reasoning_still_continues(self, loop_agent):
-        loop_agent.client.chat.completions.create.side_effect = [
-            _side_channel_length_response(
-                content="partial answer ",
-                reasoning="some leftover reasoning",
-            ),
-            _full_response("and the rest."),
-        ]
-        result = _run(loop_agent, "write me a long report")
-
-        assert result["completed"] is True
-        assert "partial answer" in (result["final_response"] or "")