…for emitDecoratorMetadata (#21966)

Fixes bug 7 of #21922.

## The bug

`@dec accessor x: T` always emitted `design:type = Object` regardless of `T`:

```ts
class Entity {
  @dec accessor name: string = "";   // tsc: design:type = String
  @dec accessor count: number = 0;   // tsc: design:type = Number
}
```

`Reflect.getMetadata("design:type", entity, "name")` returned `Object` instead of `String`. Frameworks reading `design:type` (NestJS Swagger, class-validator, TypeORM column metadata) silently degraded for every typed accessor field.

## Cause

`lower_accessor_properties` (in `enter_class`) rewrites
```ts
@dec accessor x: T = init;
```
into three members — a private storage field, a synthesized getter, and a synthesized setter — and transfers the decorators to the getter so the existing decorator pipeline picks them up. But it dropped the type annotation: the synth getter was constructed with `return_type: NONE`.

The metadata pass runs later and takes the getter branch:

```rust
if method.kind.is_get() {
    (self.serialize_type_annotation(method.value.return_type.as_ref(), ctx), None)
}
```

`return_type` is `None` because lowering discarded it, so `serialize_type_annotation` correctly fell through to `Object` for what it sees as an untyped getter.

The metadata pass's `enter_accessor_property` dispatch exists, but it never fires because the `AccessorProperty` no longer exists by the time traversal reaches the class body.

## Fix

Stop dropping the type annotation. In `lower_accessor_properties`:

```rust
let decorators = std::mem::replace(&mut accessor.decorators, ctx.ast.vec());
let type_annotation = accessor.type_annotation.take();
```

Thread `type_annotation` through `create_accessor_method` to the getter (setter passes `None`). To avoid a post-construction patch on the synthesized `MethodDefinition`, `create_class_method` now takes a `return_type: Option<ArenaBox<TSTypeAnnotation>>` parameter and threads it directly into the function's return-type slot. The other caller (`create_class_constructor_with_params`) passes `None`.

The lowered AST is now self-consistent — the synth getter for `accessor x: T` reads as a getter that returns `T`, which is what it semantically is, and the existing getter-metadata path serializes it correctly.

## Out of scope (architectural limit)

`type Uuid = string; @dec accessor x: Uuid` still emits `Object`, same as for non-accessor properties. Resolving type-alias references to their primitive constituent requires a name-based alias-resolution pass (the same one that `tsc`'s checker provides via `getTypeReferenceSerializationKind`). That's bug 1 of #21922 and a separate change from this fix; flagged here so reviewers don't mistake it for an omission.

## Test plan

- [x] Conformance fixture `oxc/metadata/accessor` covers primitives, untyped (correctly stays `Object`), `string[]` → `Array`, `static accessor`, and computed-key (`@dec accessor ["computed"]: number`)
- [x] `cargo test -p oxc_transformer` — 31 + 5 passed
- [x] `cargo run -p oxc_transform_conformance -- --filter legacy-decorators` — 73 pass / 23 fail (baseline 72/24, +1 new fixture, **0 regressions**)
- [x] `cargo run -p oxc_transform_conformance -- --exec` — accessor exec test verifies `Reflect.getMetadata` returns the correct constructor at runtime
- [x] `cargo fmt -p oxc_transformer`