docs: update GraphQL response data docs

[mateusz-pietras]

Clarify behavior of data field in response documentation as independent from errors field

Issue #, if available:

N/A

Description of changes:

Updates the GraphQLResponse.data dartdoc in amplify_core. The previous comment stated that data will be null whenever errors is non-empty, which does not match the GraphQL response format or Amplify's decoding behavior.

Per the GraphQL response format, data and errors are separate top-level fields. Field-level errors can be returned alongside a partial data payload (partial success). Amplify's GraphQLResponseDecoder preserves decoded data when the server returns both data and errors, for example:

// packages/api/amplify_api_dart/lib/src/graphql/helpers/graphql_response_decoder.dart
    // Found a JSON object to represent the model, parse it using model's fromJSON.
    T decodedData;

    if (modelType is PaginatedModelType) {
      final filter = request.variables['filter'] as Map<String, dynamic>?;
      final limit = request.variables['limit'] as int?;

      final resultNextToken = dataJson![_nextToken] as String?;
      dynamic requestForNextResult;
      // If result has nextToken property, prepare a request for the next page of results.
      if (resultNextToken != null) {
        final variablesWithNextToken = <String, dynamic>{
          ...request.variables,
          _nextToken: resultNextToken,
        };
        requestForNextResult = GraphQLRequest<T>(
          document: request.document,
          decodePath: request.decodePath,
          variables: variablesWithNextToken,
          modelType: request.modelType,
          authorizationMode: request.authorizationMode,
          apiName: request.apiName,
        );
      }
      decodedData =
          modelType.fromJson(
                dataJson!,
                limit: limit,
                filter: filter,
                requestForNextResult:
                    requestForNextResult
                        as GraphQLRequest<PaginatedResult<Model>>?,
              )
              as T;
    } else {
      decodedData = modelType.fromJson(dataJson!) as T;
    }
    return GraphQLResponse<T>(data: decodedData, errors: errors);

The updated documentation clarifies that data may be non-null when errors is non-empty, and directs callers to check hasErrors before treating the response as fully successful.

No runtime or API changes—documentation only.

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[jvh-aws]

Thanks a lot for the contribution