refactor(go): use Go generics for union types

Address review feedback from chaokunyang. Since Go 1.23+ is used,
replace interface{}-based Union with generic Union2, Union3, Union4 types.

New API:
- Union2[T1, T2], Union3[T1, T2, T3], Union4[T1, T2, T3, T4] generic types
- NewUnion2A/B, NewUnion3A/B/C, NewUnion4A/B/C/D constructors
- Match() methods for pattern matching (type-safe callbacks)
- Index(), IsFirst(), IsSecond() accessor methods
- First(), Second() value extraction methods (panics on wrong access)
- RegisterUnion2Type, RegisterUnion3Type, RegisterUnion4Type functions

Benefits:
- Compile-time type safety
- Explicit variant tracking
- Pattern matching support
- No runtime type assertions needed

Author: userzhy