fix: enforce model access control on /responses endpoint (#23481)

The /responses proxy endpoint only required authentication via
get_verified_user but did not check per-model access grants. This
allowed any authenticated user to access any model through this
endpoint, bypassing the access control system.

Extract a shared check_model_access helper into utils/access_control
and replace all inline access control blocks across openai.py and
ollama.py (7 locations) with calls to this helper. This eliminates
code duplication and prevents future policy drift between endpoints.

CWE-862: Missing Authorization
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H (6.5 Medium)

backend/open_webui/utils/access_control/__init__.py

@@ -255,3 +255,47 @@ def filter_allowed_access_grants(
         access_grants = strip_user_access_grants(access_grants)
 
     return access_grants
+
+
+def check_model_access(
+    user: UserModel,
+    model_info,
+    bypass_filter: bool = False,
+) -> None:
+    """
+    Enforce per-model read access for the given user.
+
+    Raises HTTPException(403) if the user is not authorized.
+    Does nothing if bypass_filter is True.
+
+    Args:
+        user: The authenticated user.
+        model_info: The model record from Models.get_model_by_id(),
+                    or None if the model is not registered.
+        bypass_filter: If True, skip all access checks (used by
+                       internal callers and BYPASS_MODEL_ACCESS_CONTROL).
+    """
+    from fastapi import HTTPException
+
+    if bypass_filter:
+        return
+
+    if model_info:
+        if user.role == 'user':
+            from open_webui.models.access_grants import AccessGrants
+
+            user_group_ids = {group.id for group in Groups.get_groups_by_member_id(user.id)}
+            if not (
+                user.id == model_info.user_id
+                or AccessGrants.has_access(
+                    user_id=user.id,
+                    resource_type='model',
+                    resource_id=model_info.id,
+                    permission='read',
+                    user_group_ids=user_group_ids,
+                )
+            ):
+                raise HTTPException(status_code=403, detail='Model not found')
+    else:
+        if user.role != 'admin':
+            raise HTTPException(status_code=403, detail='Model not found')