[BUG] Should not allow the hybrid query to be wrapped by another hybrid query.

[bzhangam]

What is the bug?

The hybrid query should be the top query but we miss the validation for the case that a hybrid query can be wrapped by another hybrid query.

How can one reproduce the bug?

1. Follow this doc to set up the hybrid search.
2. Do query like below:

   "_source":{
      "excludes":[
         "passage_embedding"
      ]
   },
   "query":{
      "hybrid":{
         "queries":[
            {
               "hybrid":{
                  "queries":[
                     {
                        "neural":{
                           "passage_embedding":{
                              "query_text":"wild west",
                              "model_id":"<your_model_id>",
                              "k":2
                           }
                        }
                     },
                     {
                        "match_phrase":{
                           "id":"1775029934.jpg"
                        }
                     }
                  ]
               }
            },
            {
               "match_phrase":{
                  "id":"1775029934.jpg"
               }
            }
         ]
      }
   }
}`

### What is the expected behavior?
We should see an error saying the hybrid query should be the top query and cannot be nested in another hybrid query.

### What is your host/environment?
OS: macOS v15.2
OpenSearch: main branch code

### Do you have any screenshots?
N/A

### Do you have any additional context?
N/A

[martin-gaievski]

+1 to this, for reference this is the issue when the initial check was added #498. I think this scenario was missing in that initial PR.