fix(spanner): only set snapshot isolation read timestamp for SI or optimistic txns in CloudClientExecutor (#14346)

In `CloudClientExecutor`, `CommitResponse.getSnapshotTimestamp()` was
unconditionally populated into `SpannerActionOutcome`'s
`snapshot_isolation_txn_read_timestamp` for all committing read-write
transactions.

When a standard Serializable Pessimistic transaction executes over a
multiplexed session with reads, Cloud Spanner SpanFE records an internal
snapshot timestamp and includes it in CommitResponse. Setting
snapshot_isolation_txn_read_timestamp for non-SI/non-optimistic
transactions violates systest SpotValidator invariants, causing
validation failures with: `"Snapshot read timestamp set in a txn that's
not a snapshot isolation or optimistic txn."`

This change gates setting `snapshot_isolation_txn_read_timestamp` on
(`rwTxn.repeatableRead || rwTxn.optimistic`), ensuring the field is only
populated when the transaction actually ran with snapshot isolation or
optimistic concurrency control.

Author: sakthivelmanii

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

@@ -788,7 +788,8 @@ public synchronized Status finish(Mode finishMode, OutcomeSender sender) {
               if (finishMode == Mode.COMMIT && rwTxn.runner.getCommitResponse() != null) {
                 com.google.cloud.spanner.CommitResponse commitResponse =
                     rwTxn.runner.getCommitResponse();
-                if (commitResponse.getSnapshotTimestamp() != null) {
+                if ((rwTxn.repeatableRead || rwTxn.optimistic)
+                    && commitResponse.getSnapshotTimestamp() != null) {
                   outcomeBuilder.setSnapshotIsolationTxnReadTimestamp(
                       Timestamps.toMicros(commitResponse.getSnapshotTimestamp().toProto()));
                 }