CASSANDRA-20910 Harden cross cluster node joins checks

[bereng]

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The Cassandra Jira

[arup-chauhan]

Hi @bereng,

I left a note on the Jira offering help in a scoped way. If you are open to it, I can

• audit the gossip/join admission paths here to ensure foreign nodes are rejected consistently
• strengthen CrossClusterNodesTest to assert mismatched cluster identity can never be admitted.

Happy to contribute via a small companion PR, if you prefer.