feat(download): expose aria2 disk write failure root cause at INFO level

Promote aria2 stderr lines that indicate disk write failures (e.g. the
'cause: No space left on device' line following 'Write disk cache flush
failure') from DEBUG to INFO so the root cause is visible in default logs,
including Windows-specific phrases (file locked by another process, sharing
violation). The same line is rate-limited to one INFO report per 60s window
and the report map is pruned on insert so repeated failures cannot spam the
log or grow memory. All other stderr output stays at DEBUG.

tests/services/test_aria2_downloader.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
 import asyncio
+import logging
+import time
 from pathlib import Path
 from unittest.mock import AsyncMock
 
@@ -705,3 +707,104 @@ async def test_wait_until_ready_includes_stderr_in_error():
     msg = str(exc_info.value)
     assert "code 28" in msg
     assert "ERROR: unknown option --fsync" in msg
+
+
+def test_is_disk_write_error_matches_wrapper_and_cause_lines():
+    downloader = Aria2Downloader()
+
+    assert downloader._is_disk_write_error(
+        "[ERROR] [DownloadCommand.cc:127] errorCode=9 Write disk cache flush failure index=18798"
+    )
+    assert downloader._is_disk_write_error(
+        "Exception: [AbstractDiskWriter.cc:454] errNum=28 errorCode=9 "
+        "Failed to write into the file D:\\models\\model.safetensors, "
+        "cause: No space left on device"
+    )
+    # Windows: "used by another process" = the file is locked by antivirus etc.
+    assert downloader._is_disk_write_error(
+        "Exception: ... The process cannot access the file because it is "
+        "being used by another process."
+    )
+    assert not downloader._is_disk_write_error(
+        "Download aborted. URI=https://example.com/model.safetensors"
+    )
+    assert not downloader._is_disk_write_error("")
+    assert not downloader._is_disk_write_error("bad option: --fsync")
+
+
+@pytest.mark.asyncio
+async def test_drain_stderr_promotes_disk_write_failure_to_info(caplog):
+    downloader = Aria2Downloader()
+
+    class FakeStderr:
+        def __init__(self, lines):
+            self._lines = list(lines)
+
+        def __aiter__(self):
+            return self
+
+        async def __anext__(self):
+            if not self._lines:
+                raise StopAsyncIteration
+            return self._lines.pop(0)
+
+    proc = type(
+        "Proc",
+        (),
+        {
+            "stderr": FakeStderr(
+                [
+                    b"",
+                    b"[ERROR] [WrDiskCacheEntry.cc:83] Error when trying to flush write cache",
+                    b"Exception: [AbstractDiskWriter.cc:454] errNum=28 errorCode=9 "
+                    b"Failed to write into the file D:\\models\\model.safetensors, "
+                    b"cause: No space left on device",
+                    b"Download aborted. URI=https://example.com/model.safetensors",
+                ]
+            )
+        },
+    )()
+    downloader._process = proc
+
+    with caplog.at_level(logging.DEBUG, logger="py.services.aria2_downloader"):
+        await downloader._drain_stderr()
+
+    info_records = [r for r in caplog.records if r.levelno == logging.INFO]
+    debug_records = [r for r in caplog.records if r.levelno == logging.DEBUG]
+
+    assert any(
+        "Error when trying to flush write cache" in r.message for r in info_records
+    )
+    assert any("No space left on device" in r.message for r in info_records)
+    assert any("Download aborted" in r.message for r in debug_records)
+    assert not any("Download aborted" in r.message for r in info_records)
+
+
+def test_stderr_error_rate_limited_to_one_info_per_window(caplog, monkeypatch):
+    downloader = Aria2Downloader()
+    line = "Exception: ... cause: No space left on device"
+
+    with caplog.at_level(logging.DEBUG, logger="py.services.aria2_downloader"):
+        downloader._report_stderr_error(line)
+        downloader._report_stderr_error(line)
+        monkeypatch.setattr(downloader, "_stderr_error_report", {})
+        downloader._report_stderr_error(line)
+
+    info_records = [r for r in caplog.records if r.levelno == logging.INFO]
+    debug_records = [r for r in caplog.records if r.levelno == logging.DEBUG]
+
+    assert len(info_records) == 2
+    assert len(debug_records) == 1
+    assert "repeated disk write error" in debug_records[0].message
+
+
+def test_stderr_error_report_prunes_expired_entries():
+    downloader = Aria2Downloader()
+    old_line = "Exception: ... cause: No space left on device"
+    new_line = "Exception: ... cause: Input/output error"
+    downloader._stderr_error_report[old_line] = time.monotonic() - 120.0
+
+    downloader._report_stderr_error(new_line)
+
+    assert old_line not in downloader._stderr_error_report
+    assert new_line in downloader._stderr_error_report