fix(async): offload file metadata checks

tests/test_discord_file_send.py

@@ -0,0 +1,30 @@
+import asyncio
+from pathlib import Path
+from unittest.mock import AsyncMock
+
+from app.modules.discord.discord import Discord
+
+
+def test_send_file_checks_local_file_in_threadpool(monkeypatch):
+    """Discord 文件发送应把本地文件检查移出 Discord 事件循环。"""
+    discord_client = Discord.__new__(Discord)
+    channel = AsyncMock()
+    discord_client._resolve_channel = AsyncMock(return_value=channel)
+    check_file = AsyncMock(return_value=False)
+    monkeypatch.setattr("app.modules.discord.discord.run_in_threadpool", check_file)
+
+    result = asyncio.run(
+        discord_client._send_file(
+            file_path="/tmp/missing.txt",
+            title="标题",
+            text=None,
+            userid="user-1",
+            file_name=None,
+            original_chat_id=None,
+        )
+    )
+
+    assert result == (False, None)
+    check_file.assert_awaited_once()
+    assert check_file.await_args.args[1] == Path("/tmp/missing.txt")
+    channel.send.assert_not_awaited()