Custom domain stuck in "Deleting" state — every delete attempt is Accepted (202) then Fails; cannot delete or re-add

**Describe the bug**
Adding custom domain `www.canterrastays.com` to Static Web App `swa-canterra-prod` (resource group `rg-canterra-prod`) failed at 2026-08-13T02:23 UTC with "An unknown error has occurred while adding your custom domain. Please try again later." Notably, the Validate step SUCCEEDED (HTTP 200) — the Create step itself then Failed ~10 minutes later.

The domain record is now stuck. Every delete attempt — Azure CLI (`az staticwebapp hostname delete`), direct ARM REST DELETE on `Microsoft.Web/staticSites/{site}/customDomains/{domain}` (api-version 2023-12-01), and the Azure portal — is Accepted (202), runs for ~6–10 minutes, and then transitions to **Failed** with no substatus. The portal permanently displays the record as "Deleting."

DNS is verified correct: the CNAME for www.canterrastays.com resolves to `brave-sand-01c38990f.7.azurestaticapps.net` via public resolvers (checked against 8.8.8.8).

**Activity log timeline (UTC, 2026-08-13):**
- 02:23:50 Validate Custom Domain — Succeeded (200)
- 02:23:52 Create Custom Domain — Accepted (202)
- 02:33:55 Create Custom Domain — **Failed**
- 02:39:08 Delete — Accepted (202) → 02:49:16 **Failed**
- 03:06:41 Delete (ARM REST) — Accepted (202) → 03:16:43 **Failed**
- 03:10:10 Delete — Accepted (202) → 03:16:44 **Failed**

**Correlation IDs from failed operations:**
- Create Failed: 1d37b8f4-d898-4228-a2bb-4223bea8207e
- Delete Failed: 25cbedf0-0c69-43af-bd31-057c709ca26e
- Delete Failed: f459b097-a458-41aa-a7fc-69040d5b0c72
- Delete Failed: c9908161-4c09-4c03-8a75-e9b3557484cb

**To Reproduce**
1. `az staticwebapp hostname set -n swa-canterra-prod -g rg-canterra-prod --hostname www.canterrastays.com` → fails with unknown error despite successful validation
2. `az staticwebapp hostname delete -n swa-canterra-prod -g rg-canterra-prod --hostname www.canterrastays.com --yes` → Accepted, then Fails
3. Record remains in "Deleting" state indefinitely; no client-side path can remove it

**Expected behavior**
Delete should remove the custom domain record, or surface an actionable error.

**Request**
Backend cleanup of the corrupted custom domain record so the domain can be re-added. Subscription and resource details above; happy to provide anything further.

No repository file or test is named. Start with the `az staticwebapp hostname delete` command and the ARM REST DELETE path described in the issue, then correlate their failures with the supplied activity-log IDs. Done means the stuck custom-domain record is removed so the hostname can be re-added, or an actionable error is surfaced.

Written by the indexing model from the issue text.