Reindex with wait_for_completion=false does not return deprecation warnings if security is enabled

[tvernum]

The reindex YAML tests check that a deprecation warning is returned if a sort is provided and wait_for_completion is false.

• elasticsearch/modules/reindex/src/yamlRestTest/resources/rest-api-spec/test/reindex/30_search.yml

  Lines 125 to 158 in c0f54a9

  	"Sorting deprecated wait_for_completion false":
  	- requires:
  	cluster_features: ["gte_v7.6.0"]
  	reason: "sort deprecated in 7.6"
  	test_runner_features: "warnings"
  	- do:
  	index:
  	index: test
  	id: "1"
  	body: { "order": 1 }
  	- do:
  	indices.refresh: {}
  	- do:
  	warnings:
  	- The sort option in reindex is deprecated. Instead consider using query
  	filtering to find the desired subset of data.
  	reindex:
  	refresh: true
  	wait_for_completion: false
  	body:
  	source:
  	index: test
  	sort: order
  	dest:
  	index: target
  	- set: {task: task}
  	- do:
  	tasks.get:
  	wait_for_completion: true
  	task_id: $task

This was added in #49855

However this behaviour does not work if security is enabled.

The behaviour depends on the transport action (and therefore the validator) executing within the same ThreadContext as the rest action so that theWarning response header can be returned on the http channel.

However, because security introduces asynchronous authorization steps into the action dispatch, the rest action is (sometimes) complete before the transport action executes, and the transport response headers are not returned to the client.

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)