[v24.2.x] pandaproxy/sr: protobuf rendering: refactor string rendering

[vbotbuildovich]

Backport #24992 to branch v24.2.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 v24.2.x
git checkout -b manual-backport-24992-v24.2.x-209
git cherry-pick -x efdbf2351a

git push origin manual-backport-24992-v24.2.x-209
gh pr create \
  --title "[v24.2.x] pandaproxy/sr: protobuf rendering: refactor string rendering" \
  --base "v24.2.x" \
  --label "kind/backport" \
  --head "manual-backport-24992-v24.2.x-209" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v24.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/24992 "

JIRA Link: [CORE-8992](https://redpandadata.atlassian.net/browse/CORE-8992)

[IoannisRP]

Backport #24993