Strip T | null unions in mutation engine via replace()

[FionaBronwen]

Summary

• Use MutationNode.replace() to swap T | null wrapper unions with the inner type, so downstream code sees the unwrapped type instead of a two-variant union
• Track nullability via the setNullable() state map
• Extract getNullableUnionType() from isNullableWrapper() to support the replace() call

Motivation

Previously, nullable wrapper unions (string | null, Dog | null) were kept structurally intact — the mutation called mutate() and marked the source union as nullable. This meant downstream code had to re-detect T | null structurally. With replace(), the union node is swapped out in the mutation graph and parent edges (e.g. a ModelProperty's type) update automatically. After mutation, property.type is the string scalar, not string | null.

Test plan

• Updated 3 existing nullable union tests to assert mutatedType.kind is the inner type (Scalar / Model) rather than Union
• Updated assertions to check isNullable on mutation.mutatedType instead of the source union
• Added new test: T | null on a model property is unwrapped to the inner type at the property level