Remove explicit `ModelContext` saves from `SwiftDataStore`

[cfoughty7]

The SwiftDataStore only uses the ModelContainers main context, so there should really be no need for explicit saves in the SwiftDataStore. Explicit saves can cause undesirable animation issues in views when inserting objects that drive views. The SwiftDataStore contract can be updated to make it clear that any needed explicit saves should be done by the consumer of the dependency.