IaC buckets: `region` is effectively required, undocumented, and fails silently when wrong

Question

**CLI 5.20.0 · `railway` (railway-ts-sdk) 3.3.2 · Node 24**

The [IaC reference](https://docs.railway.com/infrastructure-as-code/reference#buckets) lists bucket `region` as **optional**. In practice it's required, and getting it wrong fails in a silent, confusing way:

1. **A region-less bucket silently breaks the whole apply.** `bucket("store")` with no region never provisions ("Bucket region undefined is invalid" in the dashboard), and because it's in the same `config apply` as a `service()`, the service is left stranded at `serviceInstances = 0` too. Yet `config apply` prints `✓ Create bucket` / `✓ Create service` and exits 0 — the `environmentApplyChangeSet` mutation returns `status: "applied"` with empty `diagnostics`. Nothing tells you anything went wrong; you just find non-functional resources later.

2. **The valid region values are undiscoverable.** The docs show exactly one (`iad`, in an example) and don't list the set. The API doesn't expose it either — no region enum, and `region` isn't a field on `BucketCreateInput` / `Bucket` / `BucketInstanceDetails`. The `regions` query lists *deployment* regions (`us-east4-eqdc4a` → "Virginia", …) — which is the obvious thing to try, but buckets **reject** those. Bucket regions are the Tigris/airport-style codes (`iad`), which isn't stated anywhere. I wasted time setting `region: "us-east4-eqdc4a"` and getting the same silent failure as no region. Worse: a working bucket's own S3 credentials report `region: "auto"`, but passing `region: "auto"` back in is **also** rejected (same silent strand) — so the value you read out isn't a value you can put in.

## Repro

`package.json`: `{ "private": true, "type": "module", "devDependencies": { "railway": "3.3.2" } }`

`.railway/railway.ts`:

```ts
import { defineRailway, project, service, bucket, image } from "railway/iac";

export default defineRailway(() => {
  const store = bucket("store");                       // no region  -> silent failure
  // const store = bucket("store", { region: "iad" }); // valid region -> both deploy
  const web = service("web", { source: image("nginx:latest"), env: { FOO: "bar" } });
  return project("iac-bucket-repro", { resources: [store, web] });
});
```

```bash
npm install && railway init --name iac-bucket-repro && railway config apply --yes
```

Verified via GraphQL:

```
no region : bucketS3Credentials -> "BucketInstance not found"   |  web serviceInstances = 0
region iad: bucketS3Credentials -> { endpoint: "…storageapi.dev", … }  |  web serviceInstances = 1, SUCCESS
```