Fix nested subtypes tagged union generation (#1005)

[almarzn]

Summary

Fixes #1005 - Incorrect tagged unions when using Jackson and nested subtypes.

Problem

When using Jackson's @JsonTypeInfo and @JsonSubTypes with nested subtypes (intermediate abstract classes that also have @JsonSubTypes), the generated tagged union types referenced the intermediate interfaces directly instead of their own tagged union types.

For example, with this hierarchy:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({@Type(Mammal.class), @Type(Bird.class)})
abstract class Animal {}

@JsonSubTypes({@Type(Cat.class), @Type(Dog.class)})
abstract class Mammal extends Animal {}

@JsonSubTypes({@Type(Parrot.class), @Type(Duck.class)})
abstract class Bird extends Animal {}

Before (buggy):

type AnimalUnion = Mammal | Bird;
type MammalUnion = Cat | Dog;
type BirdUnion = Parrot | Duck;

This meant const animal: AnimalUnion = { type: "Cat", ... } would fail with a TypeScript error because Cat is not assignable to Mammal.

After (fixed):

type AnimalUnion = MammalUnion | BirdUnion;
type MammalUnion = Cat | Dog;
type BirdUnion = Parrot | Duck;

Solution

Modified ModelCompiler.createAndUseTaggedUnions to:

1. Build a map of Class<?> -> Symbol for all beans that have tagged unions before constructing the unions
2. When building union types, check if each child class has its own union alias (via the map)
3. Only replace with the union alias when the child appears as a named entry in the parent's @JsonSubTypes annotation (ensuring backward compatibility with existing behavior for intermediate types without names)

Testing

• Added new test TaggedUnionsTest.testNestedSubtypesTaggedUnion that reproduces the issue
• All 378 tests in the core module pass
• Backward compatibility verified with existing testIntermediateUnions test