…ster

The register endpoint called SetNodeLabels(req.Labels) — replace-all
semantics — so every worker re-register wiped every label not in the
worker's body. The bug existed since labels were introduced in
PR #9186 (Mar 31), but only triggered for workers that supplied
labels via --node-labels.

PR #9583 (the multi-replica refactor) added an auto-mirrored
`node.replica-slots` label to every worker's registration body, which
made `len(req.Labels) > 0` always true — turning a latent edge-case
bug into a universal one. Operators reported "labels assigned to
node do not persist": labels survived until the next worker restart,
then disappeared.

Fix: iterate req.Labels and call SetNodeLabel (upsert) for each
instead of SetNodeLabels (delete-then-recreate). Worker-managed
labels still refresh on re-register; UI-added labels survive.

Trade-off: an operator who removes a label from --node-labels won't
have it auto-removed from the DB on next register — they can clean it
via the UI. Acceptable, since the alternative (current behavior)
silently destroys operator state.

Regression test added first (TDD): RegisterNodeEndpoint registers a
node, the test simulates a UI add via SetNodeLabel, then re-registers
with a different worker label set; assertion that the UI-added label
survives. Test fails against the broken code, passes against the fix.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: claude-code:opus-4-7 [Edit] [Bash]