Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
This commit adds a new `_preview` endpoint for data frame analytics.
This allows users to see the data on which their model will be trained. This is especially useful
in the arrival of custom feature processors.
The API design is a similar to datafeed `_preview` and data frame analytics `_explain`.
