fix: retain last non-empty result set in search polling loop

DSM sends files=[] on the final finished=True poll even when results
exist in earlier rounds. Overwriting files each iteration discarded
the real results, producing a false "No files found" response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_tools_filestation.py

@@ -502,6 +502,41 @@ async def test_search_polls_until_finished(config: AppConfig) -> None:
     assert "1 match(es) found" in result
 
 
+@pytest.mark.asyncio
+async def test_search_empty_final_poll(config: AppConfig) -> None:
+    """search returns results from an earlier poll when the final finished=True poll is empty.
+
+    DSM can return files=[]] on the finishing poll even when results exist — the tool
+    must retain the last non-empty result set rather than overwriting with [].
+    """
+    client = MagicMock()
+    poll_calls = 0
+
+    async def _request(api, method, **kwargs):
+        nonlocal poll_calls
+        if method == "start":
+            return {"taskid": "t_empty_final"}
+        if method == "list":
+            poll_calls += 1
+            if poll_calls == 1:
+                # First poll: results available, not yet finished
+                return {"files": [_SEARCH_FILE], "finished": False, "total": 1}
+            # Second poll: finished, but DSM returns empty files
+            return {"files": [], "finished": True, "total": 1}
+        return {}
+
+    client.request = AsyncMock(side_effect=_request)
+
+    tools = _make_mcp_and_tools(config, client)
+
+    with patch("asyncio.sleep", new_callable=AsyncMock):
+        result = await tools["search"](path="/docker", pattern="*.yaml")
+
+    # Must surface the result from the first poll, not treat the empty final as "no results"
+    assert "1 match(es) found" in result
+    assert "/docker/app/compose.yaml" in result
+
+
 @pytest.mark.asyncio
 async def test_search_no_results(config: AppConfig) -> None:
     """search returns a friendly message when no files are found."""