fix: prevent startup crash when function/tool has null user_id (#26850)

The Function and Tool database columns declare user_id as a nullable
String column, but their Pydantic read-models required a non-null
string. A record with user_id NULL therefore raised a
pydantic ValidationError inside get_functions()/get_tools(), which run
during install_tool_and_function_dependencies() at app startup —
crashing the whole application and blocking all chat completions.

Make user_id Optional in the read/response models so such records
validate gracefully (user is already rendered as None downstream when
the id has no matching user) instead of taking down startup.


Claude-Session: https://claude.ai/code/session_01Y4RRUNq7ZUFkRWbWPkDw3m

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/models/functions.py

@@ -42,7 +42,7 @@ class FunctionMeta(BaseModel):
 
 class FunctionModel(BaseModel):
     id: str
-    user_id: str
+    user_id: str | None = None  # may be null for legacy/malformed records
     name: str
     type: str
     content: str
@@ -58,7 +58,7 @@ class FunctionModel(BaseModel):
 # --- form / schema definitions ---
 class FunctionWithValvesModel(BaseModel):
     id: str
-    user_id: str
+    user_id: str | None = None  # may be null for legacy/malformed records
     name: str
     type: str
     content: str
@@ -79,7 +79,7 @@ class FunctionWithValvesModel(BaseModel):
 
 class FunctionResponse(BaseModel):
     id: str
-    user_id: str
+    user_id: str | None = None  # may be null for legacy/malformed records
     type: str
     name: str
     meta: FunctionMeta