[v25.3.x] members_backend: drop pending raft0 additions on decommission

[vbotbuildovich]

Backport #30926 to branch v25.3.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 v25.3.x
git checkout -b manual-backport-30926-v25.3.x-40
git cherry-pick -x 04372af4f9 99b7481be1 a79b022728 e69eb3d0dc 5fe3888dfb b812baa99c

git push origin manual-backport-30926-v25.3.x-40
gh pr create \
  --title "[v25.3.x] members_backend: drop pending raft0 additions on decommission" \
  --base "v25.3.x" \
  --label "kind/backport" \
  --head "manual-backport-30926-v25.3.x-40" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.3.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/30926 "