feat: Multiplexed sessions - Fix failing test_transaction_read_and_insert_then_rollback.

Signed-off-by: Taylor Curran <taylor.curran@improving.com>

Author: currantw

tests/system/test_session_api.py

@@ -701,12 +701,11 @@ def transaction_work(transaction):
 
         if multiplexed_enabled:
             # With multiplexed sessions enabled:
-            # - Batch operations still use regular sessions (GetSession)
+            # - Batch operations use multiplexed sessions (GetSession)
             # - run_in_transaction uses regular sessions (GetSession)
-            # - Snapshot (read-only) can use multiplexed sessions (CreateMultiplexedSession)
+            # - Snapshot (read-only) re-use existing multiplexed sessions
             # Note: Session creation span may not appear if session is reused from pool
             expected_span_names = [
-                "CloudSpanner.GetSession",  # Batch operation
                 "CloudSpanner.Batch.commit",  # Batch commit
                 "CloudSpanner.GetSession",  # Transaction session
                 "CloudSpanner.Transaction.read",  # First read