ring: treat CAS no-change as success during instance registration

[Krishnachaitanyakc]

When the memberlist KV store returns "no change detected" during instance registration, the ring entry already matches what the lifecycler tried to write. The current code treats this as fatal, causing a permanent CrashLoopBackOff.

This happens when a ring entry has a corrupted last_heartbeat_at timestamp far in the future. The merge function compares timestamps and never sees forward progress (time.Now() < stored_timestamp), so every CAS retry returns "no change detected." The ruler (or any ring member) then fails to start on every restart with:

register instance in the ring: failed to CAS-update key rulers/rulers: no change detected

The fix: treat "no change detected" as a warning during registration, not a fatal error. The instance is already registered with the desired state.

Fixes grafana/loki#21733

Note: after this dskit change is merged and released, Loki will need a vendor bump to pick it up.

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Note

Medium Risk
It changes startup failure semantics for ring registration and relies on substring matching of KV errors, which could mask unrelated "no change detected" failures if that text appears elsewhere.

Overview
BasicLifecycler.registerInstance no longer fails startup when the ring KV CAS returns "no change detected". That case is treated as successful registration: the code logs a warning and continues updating local state and metrics, instead of returning an error.

This avoids a restart loop when memberlist merge sees no forward progress (e.g. a last_heartbeat_at stored ahead of wall clock) while the instance is already in the desired ring state (grafana/loki#21733). The change adds a strings import for error substring matching only on this path; other CAS errors still propagate.

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