fix(auth_utils): make header comparison case-insensitive (#12950)

If the user specified in the configuration e.g. "user_header_name:
X-OpenWebUI-User-Email", here we were looking for a dict key
"X-OpenWebUI-User-Email" when the dict actually contained
"x-openwebui-user-email".

Switch to iteration and case insensitive string comparison instead to
fix this.

This fixes customer budget enforcement when the customer ID is passed
in as a header rather than as a "user" value in the body.

litellm/proxy/auth/auth_utils.py

@@ -490,9 +490,11 @@ def get_end_user_id_from_request_body(
         custom_header_name_to_check = general_settings.get(user_id_header_config_key)
 
         if custom_header_name_to_check and isinstance(custom_header_name_to_check, str):
-            user_id_from_header = request_headers.get(custom_header_name_to_check)
-            if user_id_from_header is not None and user_id_from_header.strip():
-                return str(user_id_from_header)
+            for header_name, header_value in request_headers.items():
+                if header_name.lower() == custom_header_name_to_check.lower():
+                    user_id_from_header = header_value
+                    if user_id_from_header.strip():
+                        return str(user_id_from_header)
 
     # Check 2: 'user' field in request_body (commonly OpenAI)
     if "user" in request_body and request_body["user"] is not None:

tests/local_testing/test_auth_utils.py

@@ -169,6 +169,13 @@ def test_get_end_user_id_from_request_body_always_returns_str():
             },
             "header-priority"
         ),
+        # Test 12: user_header_name is matched case-insensitively
+        (
+            {"x-user-id": "lowercase-header-user"},
+            {"user_header_name": "X-User-ID"},
+            {"user": "body-user-456"},
+            "lowercase-header-user"
+        ),
     ]
 )
 def test_get_end_user_id_from_request_body_with_user_header_name(