fix: preserve insertion order in asset_id dedup to fix test ordering

Changed set-based dedup to order-preserving dedup in _build_asset_url_map.
Set dedup caused non-deterministic iteration order which broke
side_effect-based mock assertions in tests.

apps/api/app/api/routes/templates_editor/clips.py

@@ -93,7 +93,13 @@ def _build_asset_url_map(
         return {}
 
     # 去重：多个 clip 可能引用同一个素材
-    unique_ids = list({aid for aid in asset_ids if aid})
+    # 去重并保持顺序
+    seen: set[str] = set()
+    unique_ids = []
+    for aid in asset_ids:
+        if aid and aid not in seen:
+            seen.add(aid)
+            unique_ids.append(aid)
 
     result: dict[str, str | None] = {}
     try: