Custom domain creation fails with ResourceOperationFailure across multiple SWAs after cross-subscription move

### Describe the bug

All custom domain additions to Azure Static Web Apps fail with
"An unknown error has occurred while adding your custom domain. Please try again later."

The Activity Log shows:

{"status":"Failed","error":{"code":"ResourceOperationFailure","message":"The resource operation completed with terminal provisioning state 'Failed'."}}

`subStatus` is empty, so no actionable error surfaces to the control plane.
The domain entry then becomes stuck — even DELETE operations do not complete.

### Context

Two Static Web Apps were moved to a newly created subscription within the same
tenant shortly before this started. I suspect this may be related to the known
cross-subscription move issues affecting SWA backend state.

### What I have verified

- CNAME resolves correctly and publicly (verified via 8.8.8.8) to the exact
default hostname of the target Static Web App
- No CAA records exist on the apex or the subdomain (verified via Google DNS-over-HTTPS,
empty Answer, Status 0)
- DNS is hosted on Cloudflare, proxy disabled (DNS-only / grey cloud)
- No Azure DNS zone exists for these domains
- Free plan custom domain limit is not reached on the affected apps
- The domains are not bound to any other Azure resource

### Reproduced across every variable

| Variable | Values tried | Result |
|---|---|---|
| Domain | two unrelated domains, different registrars | fail |
| Static Web App | newly created and pre-existing apps | fail |
| Subscription | newly created subscription | fail |
| Service type | App Service (.NET 8) in the same subscription/tenant | **succeeds** |

Custom domain binding works normally on App Service using the same DNS setup.
Only Static Web Apps fail, which points to the SWA control plane rather than DNS
or tenant permissions.

### Failure identifiers

- correlationId: 8903ba11-a15e-4448-aa23-f5863f00dc00
- operationId: ad40e7d7-f101-488a-8a02-5627b74adb63
- eventTimestamp: 2026-08-12T08:16:21.9298045Z
- operationName: Microsoft.Web/staticSites/customdomains/write
- Region:Korea Central
- Plan: Free

I can provide subscription ID, resource names and affected hostnames privately
if a maintainer requests them.

### Expected behavior

Custom domain validation completes, or the operation returns a specific,
actionable error code instead of a terminal generic failure.

### Additional context

The stuck domain entries cannot be deleted, which suggests the customDomains
child resources are left in an unrecoverable provisioning state.
Please advise whether backend state can be cleared for the affected apps.

Start with the Microsoft.Web/staticSites/customdomains/write operation and the supplied correlationId and operationId, then review the Azure Activity Log for the failed custom-domain creation and stuck deletion. Done means custom-domain validation succeeds or returns a specific actionable error, and failed domain entries can be deleted or recovered.

Written by the indexing model from the issue text.