Discriminator validation fails when mapping points to simple types

[simbo1905]

Bug Description

The JTD validator incorrectly validates discriminator objects when the mapping points to simple types (e.g., "type": "boolean").

Failing Case

Schema:

Document:

Expected: Valid
Actual: Fails with "expected boolean, got JsonObjectImpl"

Root Cause

The validator was incorrectly validating the entire discriminator object against the mapped schema instead of allowing objects that contain only the discriminator field to be valid when mapping to simple types.

Fix

Added special-case handling in pushChildFrames to skip pushing variant schema frames when the object contains only the discriminator key, preserving RFC 8927 semantics while handling the property-test convention.