fix: invalid --region-id should name list-regions

Create requires `--region-id` copied from docs or from `elastic cloud serverless regions list-regions`. A typo comes back as a raw 400.

On create (and hosted `create-deployment` if the body is a region error): append `elastic cloud serverless regions list-regions` (or the hosted equivalent). Do not preflight the region list in MVP.

Acceptance: create with `--region-id not-a-region` mentions `list-regions` on stderr. `--json` includes `error.hint`.

Start at the create command's handling of region errors and the hosted create-deployment path, using the issue's invalid --region-id example to locate both flows. Verify that human-readable stderr mentions list-regions and that --json exposes error.hint, without adding a region preflight.

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