### What this PR solves

Under `experimental.onDemandWrapping`, the concatenated module-group wrapper — the single `__esm` closure holding the bodies of every module in a merged group — was always emitted as a sync arrow. When the group's entry is TLA-tainted (has top-level `await`, or awaits a TLA importee's `init` call), those `await`s land inside that closure, producing a chunk with `await` in a non-async function: a SyntaxError at load.

```js
// before
var init_entry = __esm(() => {
  ...
  await init_dep();  // SyntaxError: await outside async function
});

// after
var init_entry = __esm(async () => {
  ...
  await init_dep();
});
```

### How

One render change in `render_chunk_content` (`crates/rolldown/src/ecmascript/format/esm.rs`): emit `async () => {` when `metas[group.entry].is_tla_or_contains_tla_dependency`. This mirrors the `is_async` flag already threaded into `make_esm_wrapper_stmt` for the non-concatenated (single-module) wrapper path — the concatenated group path just never received it.

### Tests

Added `{ "strictExecutionOrder": true, "onDemandWrapping": true }` config variants to the TLA fixtures so the emitted syntax is pinned by snapshots (and executed):

- `issues/9083`
- `topics/tla/on_demand_await_call`
- `topics/tla/transitive_dep`
- `function/experimental/on_demand_wrapping/top_level_await_syntax`

Each fails on main with the sync closure and passes with this change.