[v25.2.x] CORE-14617 kafka: fix fetch session retry losing partition inclusion

[vbotbuildovich]

Backport #29304 to branch v25.2.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 v25.2.x
git checkout -b manual-backport-29304-v25.2.x-615
git cherry-pick -x b7c6970e63

git push origin manual-backport-29304-v25.2.x-615
gh pr create \
  --title "[v25.2.x] CORE-14617 kafka: fix fetch session retry losing partition inclusion" \
  --base "v25.2.x" \
  --label "kind/backport" \
  --head "manual-backport-29304-v25.2.x-615" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/29304 "