Wiebren fix/rust discriminated union child fields

[wing328]

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Summary by cubic

Fixes the Rust generator silently dropping child-specific fields in discriminated unions. Mapped children now generate boxed newtype variants like ObjectExists(Box<models::ObjectExists>) instead of inline structs built from the parent's vars.

Behavior

• The child's discriminator property defaults and skips serialization while unset, so serde no longer fails on the consumed tag key or writes the tag twice.
• Standalone use of child models is unchanged: constructors and field access keep working as before.
• Unions whose mapping names the base itself or uses an enum-typed discriminator keep the old inline variants, since there is no struct to wrap or the tag would be duplicated.
• Adds a rust reqwest sample and test spec covering nullability, enum-typed tags, and self-mapping.

^{Written for commit 41564e5. Summary will update on new commits.}