fix(channels): log DingTalk inbound-file skips instead of vanishing silently (#5683)

_receive_single_file dropped an attachment with zero log lines whenever
_download_by_code returned None (or empty bytes) - each None reason
already has its accurate line inside the download function (non-200
exchange, missing downloadUrl, oversize abort, transport failure), but
nothing tied the skip to the file being received, and the empty-bytes
case landed in the same silent branch. The caller now logs a neutral
guard line naming the file - the same shape the WeChat channel's callers
and the manager reader use since #5225.

Both the None and empty-bytes paths are pinned by caplog tests;
reverting to the silent branch turns them red. Full DingTalk suite
135 passed / 1 skipped.

backend/app/channels/dingtalk.py

@@ -629,6 +629,14 @@ class DingTalkChannel(Channel):
         """
         content = await self._download_by_code(download_code)
         if not content:
+            # Neutral on purpose: None covers several reasons (non-200
+            # download exchange, missing downloadUrl, oversize abort,
+            # transport failure), each already logged with its accurate
+            # reason inside _download_by_code — the empty-bytes case lands
+            # here too. Logging here keeps the skip observable at the
+            # receive level instead of vanishing silently (the wechat
+            # channel's callers and the manager reader use the same shape).
+            logger.warning("[DingTalk] inbound file download returned no content, skipping: file=%s", filename or "(unnamed)")
             return ""
 
         paths = get_paths()

backend/tests/test_dingtalk_channel.py

@@ -2608,3 +2608,33 @@ class TestHandlerStashesRawData:
             assert DingTalkChannel._extract_files(msg) == [{"type": "file", "download_code": "dc_doc", "filename": "a.xlsx"}]
 
         _run(go())
+
+
+class TestDingTalkDownloadGuardLogging:
+    """A None (or empty) download result used to drop the attachment with
+    zero log lines at the receive level — the accurate reason lines inside
+    _download_by_code fired, but nothing tied them to the file being
+    received. The caller now logs a neutral guard line, mirroring the
+    wechat channel's callers and the manager reader."""
+
+    def test_none_download_logs_neutral_guard_line(self, caplog):
+        async def go():
+            channel = DingTalkChannel(MessageBus(), config={})
+            channel._download_by_code = AsyncMock(return_value=None)
+            with caplog.at_level(logging.WARNING, logger="app.channels.dingtalk"):
+                result = await channel._receive_single_file("dc1", "file", "report.pdf", "t1", user_id="default")
+            assert result == ""
+
+        _run(go())
+        assert any("inbound file download returned no content" in r.message and "report.pdf" in r.message for r in caplog.records)
+
+    def test_empty_download_logs_neutral_guard_line(self, caplog):
+        async def go():
+            channel = DingTalkChannel(MessageBus(), config={})
+            channel._download_by_code = AsyncMock(return_value=b"")
+            with caplog.at_level(logging.WARNING, logger="app.channels.dingtalk"):
+                result = await channel._receive_single_file("dc2", "image", "photo.png", "t1", user_id="default")
+            assert result == ""
+
+        _run(go())
+        assert any("inbound file download returned no content" in r.message and "photo.png" in r.message for r in caplog.records)