feat: make object registration idempotent (#3752)

# What does this PR do?

objects (vector dbs, models, scoring functions, etc) have an identifier
and associated object values.

we allow exact duplicate registrations.

we reject registrations when the identifier exists and the associated
object values differ.

note: model are namespaced, i.e. {provider_id}/{identifier}, while other
object types are not

## Test Plan

ci w/ new tests

llama_stack/core/store/registry.py

@@ -96,9 +96,11 @@ class DiskDistributionRegistry(DistributionRegistry):
 
     async def register(self, obj: RoutableObjectWithProvider) -> bool:
         existing_obj = await self.get(obj.type, obj.identifier)
-        # dont register if the object's providerid already exists
-        if existing_obj and existing_obj.provider_id == obj.provider_id:
-            return False
+        if existing_obj and existing_obj != obj:
+            raise ValueError(
+                f"Object of type '{obj.type}' and identifier '{obj.identifier}' already exists. "
+                "Unregister it first if you want to replace it."
+            )
 
         await self.kvstore.set(
             KEY_FORMAT.format(type=obj.type, identifier=obj.identifier),