fix(grpc): capture loop variables in goroutine closure

Fix potential race condition by capturing loop variables 'index' and
'serviceCall' by value in the errgroup.Go() closure. Without this fix,
all goroutines could reference the same loop variables, leading to
concurrent access issues when processing gRPC service calls.

This addresses the race condition reported in cosmo router-tests when
processing recursive field resolvers with multiple levels of recursion
and aliases.

Author: asoorm

v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go

@@ -113,6 +113,9 @@ func (d *DataSource) Load(ctx context.Context, input []byte, out *bytes.Buffer)
 
 		// make gRPC calls
 		for index, serviceCall := range serviceCalls {
+			// Capture loop variables to avoid closure issues
+			index := index
+			serviceCall := serviceCall
 			errGrp.Go(func() error {
 				a := astjson.Arena{}
 				// Invoke the gRPC method - this will populate serviceCall.Output