OperationContext contains empty variables for variable with query

### What happened?

setting up the following for the handler
```go
srv.AroundOperations(func(ctx context.Context, next graphql.OperationHandler) graphql.ResponseHandler {
oc := graphql.GetOperationContext(ctx)
fmt.Printf("%+v", oc.Variables)
return next(ctx)
})
```
prints `map[]`
on the following query

```
query Query {
genConfigJson(id: 1234567)
}
```
The query succeeds and returns data in graphiql

### What did you expect?
the print statement should have printed `{"id":1234567}`

### Minimal graphql.schema and models to reproduce
```graphql
scalar Map

type Query {
genConfigJson(id: [Int!]): [Map!]
}
```

### versions
- `go run github.com/99designs/gqlgen version`?
Initially tried with `v0.17.44`. Updated to `v0.17.49` and it still happens
- `go version`?
`go version go1.22.4 linux/amd64`

问题在于，对于带有参数的查询，OperationContext.Variables 映射为空。查看 server package 中的 operation context 处理，可能重点关注 operation execution 和 variable parsing 附近的代码。测试应包括设置一个带有 AroundOperations 的 handler，并验证 variables map 包含 query argument。检查变量如何从 request 中提取并传递给 operation context。

由索引模型根据 Issue 内容生成。