🐛 Fix ParseSecretName round-trip for purposes containing hyphens

[kiarashazarnia]

/kind bug
/area bootstrap
/sig cluster-lifecycle

What this PR does / why we need it:

Some Cluster API secrets were being silently dropped during clusterctl move, causing the move graph to miss real resources. The cause is ParseSecretName failing on the APIServerEtcdClient purpose because it splits the secret name at the last hyphen and the trailing segment is not a valid purpose.

The fix replaces the LastIndex split with suffix-based matching against allSecretPurposes, mirroring HasPurposeSuffix in consts.go.

Which issue(s) this PR fixes:

Suggested Release Note"

Fix `ParseSecretName` round-trip for purposes containing hyphens by using suffix-based matching instead of `LastIndex` splitting.

Reviewer notes:

• ParseSecretName is also called from util/kubeconfig/kubeconfig.go:193, but that path only handles Kubeconfig secrets, so it never hit the bug.
• Required test also added for future regressions :) Before the fix, TestParseSecretName_RoundTrip's two APIServerEtcdClient cases ("cluster-apiserver-etcd-client" and "my-cluster-apiserver-etcd-client") returned an error because the last-hyphen split extracted "client" instead of "apiserver-etcd-client". After the fix, both parse correctly as (cluster_name, APIServerEtcdClient).

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