CASSANDRA-20910: Add repro test to prove ring mismatch rejection

[arup-chauhan]

Summary

This PR adds regression coverage for CASSANDRA-20910 to ensure foreign nodes are rejected during gossip SYN handling and cannot be admitted into membership state and is a companion PR to the existing #4618

What This Adds

• New test class: org.apache.cassandra.gms.GossipDigestSynVerbHandlerTest
• rejectsForeignClusterNameWithoutAdmission
• rejectsForeignPartitionerWithoutAdmission

Invariants Covered

• A node with mismatched cluster identity must not be admitted.
• A node with mismatched partitioner must not be admitted.

Assertions

For both mismatch cases, tests verify:

• mismatch warning is logged (ClusterName mismatch / Partitioner mismatch)
• gossip message is not processed forward (lastProcessedMessageAt unchanged)
• endpoint is not added to membership state (endpointStateMap / liveEndpoints)

Test Plan

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.GossipDigestSynVerbHandlerTest
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

[arup-chauhan]

This had overlapping work with original #4618, hence closing in favor of #4647