Add support for if/then/else conditional schemas

[cubahno]

Adds code generation support for JSON Schema if/then/else keywords, which are available in OpenAPI 3.1.

Approach

• Single branch (only then or only else): flat-merged into the parent struct as optional fields
• Both branches: generates a runtime.Conditional[T, E] union wrapper with _Then and _Else named variant types

Why Conditional instead of Either

Either uses generic .A/.B fields and .IsA()/.IsB() methods. For if/then/else this is confusing - which one is "then"? Conditional uses .Then/.Else fields and .IsThen()/.IsElse() methods, making the semantics immediately clear.

Why no If field/method on Conditional

Go reserves if as a keyword - you can't have a field or method named If. More importantly, the if schema is purely a validation predicate (e.g. "if kind equals typeA") and contributes no structural properties to code generation. It's not meaningful at the type level.

Limitations:

• The if schema is ignored - it's a validation concern, not structural
• Validation-only branches (only required, no new properties, ...) produce an empty wrapper type - the required constraint is conditional and can't be statically enforced in Go types.

Closes #74