fix: window_timeout for one-shot tasks, drop debug stderr logging

_poll_task now accepts window_timeout. For DirSize and MD5 (one-shot result
windows), if only 599 errors arrive for window_timeout seconds without ever
seeing the task alive (finished=False), return a fast "result window missed —
please retry" error instead of waiting the full 60 s. Tasks that return
finished=False at least once (large dirs, large files) are unaffected.

Also removes the stale [dsm] debug stderr.write left in client.request().

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

tests/test_tools_filestation.py

@@ -1635,8 +1635,13 @@ async def test_dir_size_retries_on_transient_599(config: AppConfig) -> None:
 
 
 @pytest.mark.asyncio
-async def test_dir_size_times_out_on_persistent_599(config: AppConfig) -> None:
-    """dir_size returns Error: after 60 s timeout when DSM keeps returning 599."""
+async def test_dir_size_window_timeout_on_persistent_599(config: AppConfig) -> None:
+    """dir_size fails fast (window_timeout=3 s) when DSM returns only 599s.
+
+    Once the window_timeout elapses without ever seeing the task running
+    (finished=False), _poll_task returns the "result window missed" error
+    rather than waiting the full 60 s.
+    """
     client = MagicMock()
 
     async def _request(api, method, version=None, params=None, **kwargs):
@@ -1651,7 +1656,7 @@ async def test_dir_size_times_out_on_persistent_599(config: AppConfig) -> None:
         result = await tools["dir_size"](path="/dead")
 
     assert result.startswith("Error:")
-    assert "timed out" in result.lower() or "60 seconds" in result
+    assert "retry" in result.lower() or "window" in result.lower() or "poll" in result.lower()
 
 
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