Amend downloadFiles loop to a Promise.all to ensure writes are complete

[EvilGenius13]

WHY are these changes introduced?

The current forEach loop in downloadFiles() doesn't await async callbacks. This causes the function to return immediately while file writes are still in progress. The CLI will show 100% progress even though files may still be writing to disk, and if there are any errors, they won't surface since the function has already completed.

WHAT is this pull request doing?

Replaces forEach(async ...) with Promise.all() to properly await all file writes. If there is an error, it will properly propagate up.

How to test your changes?

• Build the branch
• Run theme pull and ensure downloads still work and the CLI terminal output works.

You can also run this handy script to see how forEach doesn't await in the loop.

Details

``` const POKEMON_IDS = [1, 4, 7, 25, 133];

async function fetchPokemon(id) {
const response = await fetch(https://pokeapi.co/api/v2/pokemon/${id});
const data = await response.json();
return data.name;
}

function timestamp() {
return [${new Date().toISOString().split("T")[1].slice(0, -1)}];
}

async function withForEach() {
console.log(\n--- forEach ---);
console.log(${timestamp()} forEach started);

POKEMON_IDS.forEach(async (id) => {
const name = await fetchPokemon(id);
console.log(${timestamp()} fetched: ${name});
});

console.log(${timestamp()} forEach complete\n);
}

async function withPromiseAll() {
console.log(\n--- Promise.all ---);
console.log(${timestamp()} Promise.all started);

await Promise.all(
POKEMON_IDS.map(async (id) => {
const name = await fetchPokemon(id);
console.log(${timestamp()} fetched: ${name});
})
);

console.log(${timestamp()} Promise.all complete\n);
}

async function main() {
await withForEach();
// give the orphaned forEach fetches time to print
await new Promise((resolve) => setTimeout(resolve, 3000));
await withPromiseAll();
}

main().catch(console.error);

</details>
### Post-release steps

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### Measuring impact

How do we know this change was effective? Please choose one:

- [ ] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
- [ ] Existing analytics will cater for this addition
- [ ] PR includes analytics changes to measure impact

### Checklist

- [ ] I've considered possible cross-platform impacts (Mac, Linux, Windows)
- [ ] I've considered possible [documentation](https://shopify.dev) changes

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.9%	14556/18449
🟡	Branches	73.23%	7223/9863
🟡	Functions	79.09%	3704/4683
🟡	Lines	79.25%	13762/17366

Test suite run success

3777 tests passing in 1455 suites.

Report generated by 🧪jest coverage report action from efa913d