[v25.1.x] k/generator: do not use standard vectors for offset commit protocol

[vbotbuildovich]

Backport https://github.com/redpanda-data/redpanda/pull/26280 to branch v25.1.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 v25.1.x
git checkout -b manual-backport-26280-v25.1.x-785
git cherry-pick -x 1db600b34a 9cf6926842 f57ecd65fa

git push origin manual-backport-26280-v25.1.x-785
gh pr create \
  --title "[v25.1.x] k/generator: do not use standard vectors for offset commit protocol" \
  --base "v25.1.x" \
  --label "kind/backport" \
  --head "manual-backport-26280-v25.1.x-785" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/26280 "

JIRA Link: [CORE-11821](https://redpandadata.atlassian.net/browse/CORE-11821)

[CORE-11821]: https://redpandadata.atlassian.net/browse/CORE-11821?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ