Inconsistent template args behavior between model extends vs model is

[TypeSpec Playgound](https://typespec.io/playground/?c=bW9kZWwgTXlUZW1wbGF0ZTxCYXIgZXh0ZW5kcyBzdHJpbmcgfCBieXRlcz4gew0KICBmaWVsZDogQmFyOw0KfQ0KDQrIREXFN2VkyUDLV8ZLPiB7fTvKMUlzIGnUJjvKVlRlc3TGeC8vIFVuY29tbWVudCB0aGlzIG91dCBhbmQgaXQgZmFpbHMgYmVjYXVzZSBg5gCBZWRG5ACsYMRlYSDIZVBhcmFtZXRlci7HVs0tOusAyC7lAOs7xCdpc8keSXPJGH0%3D&e=%40typespec%2Fopenapi3&options=%7B%7D)

Take this example:
```tsp
model MyTemplate {
field: Bar;
}

model MyExtended extends MyTemplate {};
model MyIs is MyTemplate;

model Test {
// extendedField has type `TemplateParameter`
extendedField: MyExtended.field;
// isField has type `string`
isField: MyIs.field;
}
```

When assigning the `field` property from `MyExtended` or `MyIs` models to a property in `Test`, I expected the same results - that the type of the field is `string`. This is because in both of my models, I've passed in `string` as the template argument.

However, this is only the case when using `is`. I'm not sure if this was intentional but the behavior is surprising.