[v25.3.x] CORE-14479 schema_registry: fix avro external reference collection#29627

pgellert merged 5 commits intoredpanda-data:v25.3.xredpanda-data/redpanda:v25.3.xfrom

pgellert:backport/avro-ref-fixpgellert/redpanda:backport/avro-ref-fixCopy head branch name to clipboard