validating parts of an object for the oneOf component.

A oneOf element defines an object that validates against exactly one alternative in a list of definitions.

So to generate a preloaded form with a oneOf element, you need to validate that branch of the object against each alternative and select the right one to preload the form with the right elements.

On my own branch, I have each alternative identify itself with an element that's defined to have an enum with one member. But for general use, we need to be able to validate sub-tree against the right list of sub-schemas to get preloading done right.

The issue names no files or tests; begin by locating the oneOf validation and form-preloading flow in the JSON form generator. Done means a subtree can be checked against the available alternatives and the matching branch can be selected for preloading, including cases beyond the described enum marker approach.

Written by the indexing model from the issue text.