refactor: replace InstanceOf[T] with plain type annotations

* refactor: replace InstanceOf[T] with plain type annotations

InstanceOf[] is a Pydantic validation wrapper that adds runtime
isinstance checks. Plain type annotations are sufficient here since
the models already use arbitrary_types_allowed or the types are
BaseModel subclasses.

* refactor: convert BaseKnowledgeStorage to BaseModel

* fix: update tests for BaseKnowledgeStorage BaseModel conversion

* fix: correct embedder config structure in test