[Transform] stats can return old state information if security is enabled

[hendrikmuhs]

found via CI: #51629

If security is used and the transform user is a non-admin user _transform/_stats might return old state information after stopping the transform with wait_for_completion=true. The information is correct eventually after the index is internally refreshed via refresh_interval (1s) of the underlying lucene index.

Severity: Low, the problem only affects users(security enabled) that script transforms and rely on correct stats information.

Mitigation: Call _stats again if the expected output is wrong until the refresh has happened.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)