When learning a schema that has a linked cdc schema, we need to learn
also the cdc schema, and at the end the schema should point to the
learned cdc schema.
This is needed because the linked cdc schema is used for generating cdc
mutations, and when we process the mutations later it is assumed in some
places that the mutation's schema has a schema registry entry.
We fix a scenario where we could end up with a schema that points to a
cdc schema that doesn't have a schema registry entry. This could happen
for example if the schema is loaded before it is learned, so when we
learn it we see that it already has an entry. In that case, we need to
set the cdc schema to the learned cdc schema as well, because it could
have been loaded previously with a cdc schema that was not learned.
Fixesscylladb/scylladb#27610Closesscylladb/scylladb#27704
3a06c32749