Fix testSeqNoCollision (#52154)

We roll a new translog generation and trim operations that are above the 
global checkpoint during primary-replica resync. If the initOperations
is empty, then the stale operation on the replica2 will be discarded as
it is the only operation in a translog file (since #51905 where we 
started using the local checkpoint to calculate the minimum required
translog generation for recovery). Otherwise, the stale op will be
retained along with initOperations but will be skipped in snapshots.

Relates #51905
Closes #52148

Author: dnhatn

server/src/test/java/org/elasticsearch/index/replication/IndexLevelReplicationTests.java

@@ -536,7 +536,6 @@ public void testRequestFailureReplication() throws Exception {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/52148")
     public void testSeqNoCollision() throws Exception {
         try (ReplicationGroup shards = createGroup(2)) {
             shards.startAll();
@@ -565,19 +564,21 @@ public void testSeqNoCollision() throws Exception {
                 assertThat(snapshot.next(), nullValue());
                 assertThat(snapshot.skippedOperations(), equalTo(0));
             }
-            // Make sure that replica2 receives translog ops (eg. op2) from replica1
-            // and does not overwrite its stale operation (op1) as it is trimmed.
             logger.info("--> Promote replica1 as the primary");
             shards.promoteReplicaToPrimary(replica1).get(); // wait until resync completed.
+            // We roll a new translog generation and trim operations that are above the global checkpoint during primary-replica resync.
+            // If the `initOperations` is empty, then the stale operation on the replica2 will be discarded as it is the only operation
+            // in a translog file. Otherwise, the stale op will be retained along with initOperations but will be skipped in snapshot.
+            int staleOps = initOperations.isEmpty() ? 0 : 1;
             shards.index(new IndexRequest(index.getName()).id("d2").source("{}", XContentType.JSON));
             final Translog.Operation op2;
             try (Translog.Snapshot snapshot = getTranslog(replica2).newSnapshot()) {
-                assertThat(snapshot.totalOperations(), equalTo(initDocs + 2));
+                assertThat(snapshot.totalOperations(), equalTo(initDocs + 1 + staleOps));
                 op2 = snapshot.next();
                 assertThat(op2.seqNo(), equalTo(op1.seqNo()));
                 assertThat(op2.primaryTerm(), greaterThan(op1.primaryTerm()));
                 assertThat("Remaining of snapshot should contain init operations", snapshot, containsOperationsInAnyOrder(initOperations));
-                assertThat(snapshot.skippedOperations(), equalTo(1));
+                assertThat(snapshot.skippedOperations(), equalTo(staleOps));
             }
 
             // Make sure that peer-recovery transfers all but non-overridden operations.