CASSANDRA-20910: Harden final alive admission for foreign nodes

[arup-chauhan]

Summary

Follow-up hardening for CASSANDRA-20910 on 4.1.

This PR adds a regression test and a minimal final-admission guard so a foreign-identity endpoint cannot be marked alive in the realMarkAlive path.

This is a companion, stacked follow-up to #4618

What This PR Changes

• Adds testRealMarkAliveRejectsForeignClusterNode in GossiperTest
• Adds an early return in Gossiper.realMarkAlive(...) when maybeBelongsInCluster(...) is false

Invariant Enforced

• A peer with mismatched cluster identity must never transition into live membership, including at final alive admission.

Test Plan

Using JDK 11:

JDK11_HOME=$(jenv prefix 11.0.30)
JAVA_HOME="$JDK11_HOME" PATH="$JAVA_HOME/bin:$PATH" ant -Duse.jdk11=true testsome -Dtest.name=org.apache.cassandra.gms.GossiperTest

Validation performed:

- Before fix: new test fails (Foreign endpoint should not be admitted to live endpoints)
- After fix: GossiperTest passes