fix explain action on query rewrite

[fen-qin]

Description

There is an exception when call explain in "by_doc_id" mode. Response looks like this:

{
    "error": {
        "root_cause": [
            {
                "type": "query_shard_exception",
                "reason": "failed to create query: async actions are left after rewrite",
                "index": "my-nlp-index-1",
                "index_uuid": "I6_pzGG3QBWw0B6KeOJ1pA"
            }
        ],
        "type": "query_shard_exception",
        "reason": "failed to create query: async actions are left after rewrite",
        "index": "my-nlp-index-1",
        "index_uuid": "I6_pzGG3QBWw0B6KeOJ1pA",
        "caused_by": {
            "type": "illegal_state_exception",
            "reason": "async actions are left after rewrite"
        }
    },
    "status": 400
}

The error were caught from TransportExplainAction -> Rewriteable.java because there still be asyncActions left when the flag assertNoAsyncTasks returned as true.

The conflict is down to:

1. The NeuralQueryBuilder or TermsQueryBuilder doRewrite method introduces asynchronous actions via queryRewriteContext.registerAsyncAction. So it then returns a context with potentially unresolved async tasks.

2. Rewritable.rewrite then checksassertNoAsyncTasks flag, throws the exception if there exists any async tasks left.

Fix in this PR is resolve this conflict:

• by moving the query rewrite to coordinator - TransportExplainAction.
• verify the changes from local using TermsQueryBuilder
  • Before fix
  • 
  • After fix
  • 

Related Issues

Resolves #1126

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 3b159cd: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?