api: Include stack trace when reporting MalformedServerResponseException

Fixes: zulip#1083

Author: lakshya1goel

lib/api/core.dart

@@ -177,7 +177,7 @@ class ApiConnection {
       Error.throwWithStackTrace(
         MalformedServerResponseException(
           routeName: routeName, httpStatus: httpStatus, data: json,
-          causeException: exception),
+          causeException: exception, causeStackTrace: stackTrace),
         stackTrace);
     }
   }

lib/api/exception.dart

@@ -149,13 +149,18 @@ class MalformedServerResponseException extends ServerException {
   /// in order to preserve the underlying exception's stack trace, which
   /// may be more informative than the exception object itself.
   final Object? causeException;
+  final StackTrace? causeStackTrace;
 
   MalformedServerResponseException({
     required super.routeName,
     required super.httpStatus,
     required super.data,
     this.causeException,
-  }) : super(message: causeException == null
+    this.causeStackTrace
+  }) : assert((causeException == null && causeStackTrace == null) ||
+              (causeException != null && causeStackTrace != null),
+         'causeException and causeStackTrace must either both be null or both be non-null'),
+       super(message: causeException == null
          ? GlobalLocalizations.zulipLocalizations
             .errorMalformedResponse(httpStatus)
          : GlobalLocalizations.zulipLocalizations

test/api/core_test.dart

@@ -425,7 +425,8 @@ void main() {
     } catch (e, st) {
       check(e).isA<MalformedServerResponseException>()
         ..causeException.isA<DistinctiveError>()
-        ..message.contains("something is wrong");
+        ..message.contains("something is wrong")
+        ..causeStackTrace.toString().contains("distinctivelyNamedFromJson");
       check(st.toString()).contains("distinctivelyNamedFromJson");
     }
   });

test/api/exception_checks.dart

@@ -28,4 +28,5 @@ extension Server5xxExceptionChecks on Subject<Server5xxException> {
 
 extension MalformedServerResponseExceptionChecks on Subject<MalformedServerResponseException> {
   Subject<Object?> get causeException => has((e) => e.causeException, 'causeException');
+  Subject<String> get causeStackTrace => has((e) => e.causeStackTrace.toString(), 'causeStackTrace');
 }

test/api/exception_test.dart

@@ -25,4 +25,6 @@ void main() {
   });
 
   // NetworkException.toString: see "API network errors" test in core_test.dart
+
+  // MalformedServerResponseException: see "malformed API success responses: exception preserves details" test in core_test.dart
 }