All deployments fail with 'An unknown exception has occurred' at upload phase since 13 Aug - partition .7

### Describe the bug

Since approximately **2026-08-13 12:00 UTC**, every deployment to our Static Web Apps fails during the content upload phase. StaticSitesClient reports only `An unknown exception has occurred` immediately after `Determined default file to be: index.html`. Serving of already-deployed content is unaffected.

Symptoms match #1750 (slice-level upload failures, same message, clean resources affected) but on a different partition: our apps' hostnames are on `.7.azurestaticapps.net`.

### Evidence this is service-side, not configuration

- Fails from GitHub Actions (`Azure/static-web-apps-deploy@v1`, Linux) AND from the SWA CLI locally (Windows; StaticSitesClient `stable` 689a6c1f and `latest` d3c91588)
- Fails for a one-file hello-world app with no config and no API
- Fails identically on two different static web apps in the subscription (`tfp-portfolio-report`, `tfp-swa-spike`)
- Deployment token rotated multiple times, fetched fresh via az at run time - no change
- Fails to the production environment AND to a newly-named preview environment
- Subscription Enabled; no Service Health events
- Production environment now stuck in `Uploading` since 2026-08-14 08:22 UTC after a failed attempt

### Deployment IDs for backend trace

- `3765945a-66df-4962-a0ec-e950511d7024` (2026-08-14 ~08:2x UTC, production, tfp-portfolio-report)
- `b031203b-c7ce-4644-aa53-cb10f84cce3b` (2026-08-14 ~09:3x UTC, direct StaticSitesClient run)

### Timeline

Last successful production deployment 2026-08-11; first observed failure 2026-08-13 ~12:00 UTC; all subsequent GitHub runs fail within 14-36 seconds.

Please investigate the deployment backend for partition .7 and clear the stuck Uploading deployment on tfp-portfolio-report/default.

No repository files or tests are named. Start by tracing deployment IDs 3765945a-66df-4962-a0ec-e950511d7024 and b031203b-c7ce-4644-aa53-cb10f84cce3b, then compare the upload failure with #1750 and partition .7. Done means the backend cause is addressed and the stuck Uploading deployment is cleared.

Written by the indexing model from the issue text.