fix(spanner): fix flakiness in testCreateSessionDeadlineExceeded (#12944)

The test `testCreateSessionDeadlineExceeded` in
`MultiplexedSessionDatabaseClientMockServerTest` was flaky because it
used a non-sticky exception for simulated `DEADLINE_EXCEEDED` on
`CreateSession`. This exception could be consumed by background session
creation tasks before the test could verify it.

This change uses `ofStickyException` to ensure the exception persists
until explicitly cleared by `mockSpanner.removeAllExecutionTimes()`,
making the test robust against concurrent background activity.

Fixes: #12890

Author: sakthivelmanii

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/MultiplexedSessionDatabaseClientMockServerTest.java

@@ -105,7 +105,7 @@ public void createSpannerInstance() {
   public void testCreateSessionDeadlineExceeded() {
     // Simulate a problem with the CreateSession RPC making it slow.
     mockSpanner.setCreateSessionExecutionTime(
-        SimulatedExecutionTime.ofException(Status.DEADLINE_EXCEEDED.asRuntimeException()));
+        SimulatedExecutionTime.ofStickyException(Status.DEADLINE_EXCEEDED.asRuntimeException()));
 
     Spanner testSpanner =
         SpannerOptions.newBuilder()