[v24.3.x] [CORE-8999] schema_registry/protobuf: Add test for legacy_map

[vbotbuildovich]

Backport #25001 to branch v24.3.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v24.3.x
git checkout -b manual-backport-25001-v24.3.x-952
git cherry-pick -x d33def4a19

git push origin manual-backport-25001-v24.3.x-952
gh pr create \
  --title "[v24.3.x] [CORE-8999] schema_registry/protobuf: Add test for legacy_map" \
  --base "v24.3.x" \
  --label "kind/backport" \
  --head "manual-backport-25001-v24.3.x-952" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v24.3.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/25001 "

JIRA Link: [CORE-9001](https://redpandadata.atlassian.net/browse/CORE-9001)