Windows App SDK Push: documented PFN mapping mailbox bounces, packaged Store app gets WNS 403

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### Summary

We need PFN -> Entra AppId/ObjectId mapping for a production Store-distributed packaged WinUI 3 / Windows App SDK app. The documented mapping process currently appears unreachable because `Win_App_SDK_Push@microsoft.com` rejects mail.

### App type

- Production packaged WinUI 3 / Windows App SDK desktop app
- Distributed through Microsoft Store / Partner Center
- Intended for third-party customer users, not only users inside our organization
- Uses the Windows App SDK push notification flow, not legacy UWP WNS credentials

### Public app identifiers

- Store product ID: `9P64ZMCCCMNR`
- PFN: `27369LabVoiceInc.LabVoice_04arp211gq58w`
- Package identity name: `27369LabVoiceInc.LabVoice`

We can provide the Entra AppId/ObjectId privately to Microsoft staff for the mapping request, but we would rather not publish those identifiers in a public GitHub issue.

### What works

- `PushNotificationManager.Register()` succeeds.
- `CreateChannelAsync(...)` succeeds.
- A WNS channel URI is returned.
- Backend token acquisition for `https://wns.windows.com/.default` succeeds using a dedicated multi-tenant Entra app.

### Failure

Sending a push notification to the returned Windows App SDK channel URI fails with `403 Forbidden`.

This looks consistent with the PFN not being mapped to the Entra AppId/ObjectId yet.

### Documented mapping process is blocked

The current quickstart says packaged apps should request mapping by emailing `Win_App_SDK_Push@microsoft.com` with PFN, AppId, and ObjectId.

That mailbox bounces:

```text
550 5.4.1 Recipient address rejected: Access denied
```

### Azure route also appears gated

We registered `Microsoft.WindowsPushNotificationServices` on our Azure subscription. Registration succeeded, but the provider exposes only `checkNameAvailability`, no create/list/update operations or usable resource types.

A public research pass suggests the actual ARM type may be `Microsoft.WindowsPushNotificationServices/registrations`, but we found no public REST schema or usable portal/ARM creation path.

### Request

Could the Windows App SDK team please clarify the current production process for PFN -> Entra AppId/ObjectId mapping?

Specifically, please either:

1. process this PFN mapping through a private channel,
2. provide the replacement mapping contact/process,
3. fix or replace the documented mailbox, or
4. document the Azure/Partner Center self-service route if one exists.

Official doc currently pointing to the bouncing mailbox:
https://learn.microsoft.com/en-us/windows/apps/develop/notifications/push-notifications/push-quickstart

Start with the Windows App SDK push quickstart linked in the issue, then review the reported mailbox bounce and the Azure provider capabilities described above. Done means the production PFN-to-Entra mapping process, replacement contact, or self-service route is clarified and the documentation is corrected.

Written by the indexing model from the issue text.