fix(agent): preserve length-recovered output

tests/agent/test_runner_injections.py

@@ -352,6 +352,42 @@ async def test_checkpoint2_injects_after_final_response_with_resuming_stream():
     assert stream_end_calls[-1] is False
 
 
+@pytest.mark.asyncio
+async def test_injected_followup_starts_new_length_recovery_chain():
+    """Recovered content from the prior answer must not prefix a follow-up reply."""
+    from nanobot.agent.runner import AgentRunner
+    from nanobot.bus.events import InboundMessage
+
+    provider = MagicMock()
+    provider.chat_with_retry = AsyncMock(side_effect=[
+        LLMResponse(content="first", finish_reason="length"),
+        LLMResponse(content="second", finish_reason="stop"),
+        LLMResponse(content="follow-up answer", finish_reason="stop"),
+    ])
+    tools = MagicMock()
+    tools.get_definitions.return_value = []
+
+    injection_queue = asyncio.Queue()
+    inject_cb = _make_injection_callback(injection_queue)
+    await injection_queue.put(
+        InboundMessage(channel="cli", sender_id="u", chat_id="c", content="follow-up question")
+    )
+
+    runner = AgentRunner()
+    result = await runner.run(make_run_spec(provider,
+        initial_messages=[{"role": "user", "content": "give a long answer"}],
+        tools=tools,
+        model="test-model",
+        max_iterations=5,
+        max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
+        injection_callback=inject_cb,
+    ))
+
+    assert result.had_injections is True
+    assert result.final_content == "follow-up answer"
+    assert provider.chat_with_retry.await_count == 3
+
+
 @pytest.mark.asyncio
 async def test_checkpoint2_preserves_final_response_in_history_before_followup():
     """A follow-up injected after a final answer must still see that answer in history."""