CCR: Tighten requesting range check on leader

This commit clarifies the origin of the global checkpoint that the
following shard uses and replaces illegal_state_exc E by an assertion.

Relates #30980

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java

@@ -236,13 +236,11 @@ protected Response shardOperation(Request request, ShardId shardId) throws IOExc
             IndexService indexService = indicesService.indexServiceSafe(request.getShard().getIndex());
             IndexShard indexShard = indexService.getShard(request.getShard().id());
             final long indexMetaDataVersion = clusterService.state().metaData().index(shardId.getIndex()).getVersion();
-            // The following shard generates the request based on the global checkpoint which may not be synced to all leading copies.
-            // However, this guarantees that the requesting range always be below the local-checkpoint of any leading copies.
-            final long localCheckpoint = indexShard.getLocalCheckpoint();
-            if (localCheckpoint < request.minSeqNo || localCheckpoint < request.maxSeqNo) {
-                throw new IllegalStateException("invalid request from_seqno=[" + request.minSeqNo + "], " +
-                    "to_seqno=[" + request.maxSeqNo + "], local_checkpoint=[" + localCheckpoint + "], shardId=[" + shardId + "]");
-            }
+            // The following shard generates this request based on the global checkpoint on the primary copy on the leader.
+            // Although this value might not have been synced to all replica copies on the leader, the requesting range
+            // is guaranteed to be at most the local-checkpoint of any shard copies on the leader.
+            assert request.maxSeqNo <= indexShard.getLocalCheckpoint() : "invalid request from_seqno=[" + request.minSeqNo + "]," +
+                " to_seqno=[" + request.maxSeqNo + "], local_checkpoint=[" + indexShard.getLocalCheckpoint() + "]";
             final Translog.Operation[] operations =
                 getOperationsBetween(indexShard, request.minSeqNo, request.maxSeqNo, request.maxTranslogsBytes);
             return new Response(indexMetaDataVersion, operations);