Memberlist cas error code false positive

[SungJin1212]

getCasErrorCode used a direct type assertion to detect IsOperationAborted(), which fails when the error is wrapped by memberlist's trySingleCas:

  fmt.Errorf("fn returned error: %w", err)          // 1st wrap
  fmt.Errorf("failed to CAS-update key %s: %w", ...) // 2nd wrap

This caused ReplicasNotMatchError (normal HA deduplication) to be reported as status_code="500" in cortex_kv_request_duration_seconds when using memberlist as the HA tracker KV store, triggering false-positive CortexKVStoreFailure alerts.

Note: consul and etcd are unaffected because they return the callback error directly without wrapping (return err), so the direct type assertion worked correctly for those backends.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[friedrichg]

Thanks!