any() ergonomics and union tuple signature

## Objective
Enhance the `any()` builder to support an additional signature that yields a union of provided types instead of a tuple, e.g., `anyUnion(S1, S2, ...)`.

### Details
- Keep current `any()` behavior for tuples, but add `anyUnion()` (or similar) for union use-cases.
- Document guidance on when to use which.
- Ensure backward compatibility.

Parent Epic: schema-inference-epic