api [nfc]: Assert ZulipApiException.data is free of redundant keys

These three keys appear in the server's JSON for error responses, but
get pulled out into their own dedicated fields in ZulipApiException.
(See the field's doc, and the constructor's one non-test call site.)

The assertion is useful for tests, for keeping test data realistic.
Fix the two test cases that had missed this nuance.

Author: gnprice

lib/api/exception.dart

@@ -81,7 +81,10 @@ class ZulipApiException extends HttpException {
     required this.code,
     required this.data,
     required super.message,
-  }) : assert(400 <= httpStatus && httpStatus <= 499);
+  }) : assert(400 <= httpStatus && httpStatus <= 499),
+       assert(!data.containsKey('result')
+           && !data.containsKey('code')
+           && !data.containsKey('msg'));
 
   @override
   String toString() {

test/model/actions_test.dart

@@ -105,7 +105,7 @@ void main() {
       final exception = ZulipApiException(
         httpStatus: 401,
         code: 'UNAUTHORIZED',
-        data: {"result": "error", "msg": "Invalid API key", "code": "UNAUTHORIZED"},
+        data: {},
         routeName: 'removeEtcEtcToken',
         message: 'Invalid API key',
       );
@@ -174,7 +174,7 @@ void main() {
         ..prepare(exception: ZulipApiException(
             httpStatus: 401,
             code: 'UNAUTHORIZED',
-            data: {"result": "error", "msg": "Invalid API key", "code": "UNAUTHORIZED"},
+            data: {},
             routeName: 'removeEtcEtcToken',
             message: 'Invalid API key',
           ));