Cosmos: cleanup CosmosModelValidator to use helper methods rather than FindAnnotation in various validation methods

Per https://github.com/dotnet/efcore/pull/35868#discussion_r2023787835

example: we should be doing 

```cs
index.IsFullTextIndex() == true
```

rather than:

```cs
index.FindAnnotation(CosmosAnnotationNames.FullTextIndex) != null
```
