Fix duplicate discriminator field in object variants

[treeform]

Summary

• Detect duplicate discriminator fields with conflicting values during JSON parsing of object variants.
• In debug mode, catches Nim's FieldDefect; in release mode, compares the discriminator before and after parsing.
• Both paths raise a clear JsonError instead of silently corrupting the variant.

Test plan

• Added tests for both ValueNode and RefNode with conflicting duplicate discriminator JSON.