node: Skip equal objects in LocalNodeStore.Update()

[jiashengz]

Don't emit (and wake up observers) when a mutator produces no change to the local node. This prevents redundant downstream work -- most importantly the nodediscovery CiliumNode resource writes that are triggered on every local node emission. During a Kubernetes apiserver outage those writes fail and the retry path ends in logging.Fatal, crashing the agent. Skipping no-op updates keeps the agent from re-arming that fatal path when nothing actually changed.

This is a backport of the dedup behavior from upstream's StateDB-based LocalNodeStore, adapted to the 1.18 stream-based store.

Upstream-reference: cilium#41294

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Fixes: #issue-number

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