refactor(memory): tighten metadata field types and clean up descriptions

- Use Literal['set', 'remove'] for MetadataFieldChange.action instead of str
- Simplify field_path description to reflect current schema
- Remove redundant isinstance check in extract_user_metadata_task

api/app/core/memory/models/metadata_models.py

@@ -4,7 +4,7 @@ Independent from triplet_models.py - these models are used by the
 standalone metadata extraction pipeline (post-dedup async Celery task).
 """
 
-from typing import List, Optional
+from typing import List, Literal, Optional
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -35,9 +35,9 @@ class MetadataFieldChange(BaseModel):
 
     model_config = ConfigDict(extra="ignore")
     field_path: str = Field(
-        description="字段路径，用点号分隔，如 'profile.role'、'knowledge_tags'、'behavioral_hints.tone_preference'"
+        description="字段路径，用点号分隔，如 'profile.role'、'profile.expertise'"
     )
-    action: str = Field(
+    action: Literal["set", "remove"] = Field(
         description="操作类型：'set' 表示新增或修改，'remove' 表示移除"
     )
     value: Optional[str] = Field(

api/app/tasks.py

@@ -3157,8 +3157,6 @@ def extract_user_metadata_task(
                         leaf = parts[-1]
 
                         current_list = target.get(leaf, [])
-                        if not isinstance(current_list, list):
-                            current_list = []
 
                         if action == "set":
                             if value not in current_list: