[v25.1.x] rpk: fix concurrent map writes in proto serde encodefn

[vbotbuildovich]

Backport #26619 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-26619-v25.1.x-30
git cherry-pick -x d40a21152b c5276c8e78

git push origin manual-backport-26619-v25.1.x-30
gh pr create \
  --title "[v25.1.x] rpk: fix concurrent map writes in proto serde encodefn" \
  --base "v25.1.x" \
  --label "kind/backport" \
  --head "manual-backport-26619-v25.1.x-30" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/26619 "

JIRA Link: [CORE-12730](https://redpandadata.atlassian.net/browse/CORE-12730)

[CORE-12730]: https://redpandadata.atlassian.net/browse/CORE-12730?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ