fix: drop extra='allow' on FolderForm and FolderUpdateForm (#23648)

* fix: drop extra='allow' on FolderForm and FolderUpdateForm

These request models were configured to accept arbitrary extra fields,
which were then merged into the folder row via form_data.model_dump().
In insert_new_folder the server-assigned user_id is placed before the
form spread, so a client-supplied user_id in the request body would
override it and the folder would be persisted against another account.

Strictly typed inputs are the correct shape for these endpoints — the
client has no legitimate reason to send fields beyond the declared
ones, and dropping extra='allow' closes the mass-assignment sink at
the validation layer instead of relying on every callsite to merge
fields in the right order.

* fix: reject unknown fields on FolderForm and FolderUpdateForm

Address review feedback: dropping extra='allow' fell back to Pydantic
v2's default extra='ignore', which only silently drops unknown fields
instead of rejecting them. The intent for these request models is a
strict input contract — fail fast when a client sends anything the
server does not expect — so explicitly set extra='forbid'. This also
makes the hardening visible in the form definition rather than implicit
in the default.

backend/open_webui/models/folders.py

@@ -74,14 +74,14 @@ class FolderForm(BaseModel):
     data: Optional[dict] = None
     meta: Optional[dict] = None
     parent_id: Optional[str] = None
-    model_config = ConfigDict(extra='allow')
+    model_config = ConfigDict(extra='forbid')
 
 
 class FolderUpdateForm(BaseModel):
     name: Optional[str] = None
     data: Optional[dict] = None
     meta: Optional[dict] = None
-    model_config = ConfigDict(extra='allow')
+    model_config = ConfigDict(extra='forbid')
 
 
 class FolderTable: