Refine FrontDoor service spec

The FrontDoor TypeSpec at `specification/frontdoor/resource-manager/Microsoft.Network/FrontDoor` defines several local base models that duplicate shapes already provided by the standard ARM common-types / `Azure.ResourceManager.Foundations` library. As part of the .NET SDK migration to the new management-plane generator, we had to paper over this mismatch with `@@alternateType` decorators in `client.tsp` to produce the correct C# base classes:

- `Microsoft.Network.Resource` → `Azure.ResourceManager.Foundations.TrackedResource`
- `ResourcewithSettableName` → `Azure.ResourceManager.Foundations.TrackedResource`
- `BasicResource` → `Azure.ResourceManager.Foundations.ProxyResource`
- `BasicResourceWithSettableIDName` → renamed via `@@clientName` to `FrontDoorResourceData` (acts like a `TrackedResource` with a settable name)

Some or all of these local models should be replaced with the standard ARM resource base models (`TrackedResource` / `ProxyResource` / common-types `Resource`) so that:

1. The spec itself describes the resources correctly instead of relying on language-specific `@@alternateType` workarounds.
2. All SDK languages (not just .NET) benefit from the correct base types and inheritance.
3. The `client.tsp` customization surface shrinks, reducing maintenance burden.

Any local properties these models add (e.g. settable `name`, extra `id` visibility) should be reviewed — most are either already provided by the standard base types or can be expressed via the standard decorators (`@visibility`, `@@clientName`, etc.).

Context: .NET SDK migration PR for `Azure.ResourceManager.FrontDoor`.