chore: Refactor SI options in CloudClientExecutor (#13697)

chore: Refactor SI options handling to include read lock mode and
isolation level based on conditions.

---------

Co-authored-by: Sakthivel Subramanian <179120858+sakthivelmanii@users.noreply.github.com>

Author: gyang-google

java-spanner/google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudClientExecutor.java

@@ -394,15 +394,14 @@ public void startRWTransaction() throws Exception {
                   List<TransactionOption> transactionOptions = new ArrayList<>();
                   if (repeatableRead) {
                     transactionOptions.add(Options.isolationLevel(IsolationLevel.REPEATABLE_READ));
-                  } else {
-                    transactionOptions.add(Options.isolationLevel(IsolationLevel.SERIALIZABLE));
+                    transactionOptions.add(
+                        Options.readLockMode(
+                            optimistic ? ReadLockMode.OPTIMISTIC : ReadLockMode.PESSIMISTIC));
                   }
                   if (!repeatableRead && optimistic) {
+                    transactionOptions.add(Options.isolationLevel(IsolationLevel.SERIALIZABLE));
                     transactionOptions.add(Options.readLockMode(ReadLockMode.OPTIMISTIC));
                   }
-                  if (repeatableRead && !optimistic) {
-                    transactionOptions.add(Options.readLockMode(ReadLockMode.PESSIMISTIC));
-                  }
                   runner =
                       dbClient.readWriteTransaction(
                           transactionOptions.toArray(
@@ -786,11 +785,13 @@ public synchronized Status finish(Mode finishMode, OutcomeSender sender) {
               if (rwTxn.getTimestamp() != null) {
                 outcomeBuilder.setCommitTime(rwTxn.getTimestamp());
               }
-              if (finishMode == Mode.COMMIT
-                  && rwTxn.runner.getCommitResponse().getSnapshotTimestamp() != null) {
-                outcomeBuilder.setSnapshotIsolationTxnReadTimestamp(
-                    Timestamps.toMicros(
-                        rwTxn.runner.getCommitResponse().getSnapshotTimestamp().toProto()));
+              if (finishMode == Mode.COMMIT && rwTxn.runner.getCommitResponse() != null) {
+                com.google.cloud.spanner.CommitResponse commitResponse =
+                    rwTxn.runner.getCommitResponse();
+                if (commitResponse.getSnapshotTimestamp() != null) {
+                  outcomeBuilder.setSnapshotIsolationTxnReadTimestamp(
+                      Timestamps.toMicros(commitResponse.getSnapshotTimestamp().toProto()));
+                }
               }
               clear();
             }