fix: rewrite self/static/parent in variadic parameter types

[chadmandoo]

What

A self-typed variadic parameter fails to compile:

final class Bag {
    public function __construct(public string $x) {}
    public static function concat(self ...$items): self { /* ... */ }   // error
}

→ Cannot use 'self' as a type outside of a class

self/static/parent already work everywhere else — regular parameter types, return types, property types.

Why

substitute_relative_class_types_in_methods rewrites the relative class types self/static/parent to the concrete enclosing class before type resolution. It walks each method's regular parameters (method.params) and its return type (method.return_type) — but not the variadic parameter's type (method.variadic_type), which is a separate field. So self on a variadic param survives to resolve_type_expr, which rejects a bare self as "outside of a class" (relative types are expected to have been rewritten already).

How

Rewrite method.variadic_type alongside the regular params and the return type. Three lines.

Tests

tests/codegen/oop/relative_types.rs — new regression test test_self_typed_variadic_param: a static concat(self ...$items): self compiles and runs (Bag::concat(...) → abc). Existing relative-types and variadic codegen tests still pass.

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