fix(distributed): scope Upgrade All to nodes that have the backend installed (#9678)

In distributed mode the React UI's "Upgrade All" button fanned every
detected outdated backend out to every healthy backend node, including
nodes that never had that backend installed. On heterogeneous clusters
this surfaced as platform errors (e.g. mac-mini-m4 asked to upgrade
cpu-insightface-development, which has no darwin/arm64 variant) and left
forever-retrying pending_backend_ops rows.

DistributedBackendManager.UpgradeBackend now queries ListBackends()
first, builds the target node-ID set from SystemBackend.Nodes, and only
fans out to those nodes — every per-node primitive
(adapter.InstallBackend, the pending-ops queue, BackendOpResult) is
unchanged. enqueueAndDrainBackendOp gains an optional targetNodeIDs
allowlist; Install/Delete keep their fan-to-everyone semantics by
passing nil. If no node reports the backend installed, UpgradeBackend
now returns a clear "not installed on any node" error instead of
producing a stuck queue.

Adds Ginkgo coverage for the smart fan-out: backend on a subset of
nodes goes only to those nodes; backend on no node returns the new
error and never sends a NATS install request.


Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>