chore: handled race condition in stateless query integration test (#4045)

* fix: handled race condition in stateless query integration test

The testTableResultJobIdAndQueryId test was failing intermittently on slower networks. The test strictly asserted that Job ID must be null for stateless queries.

However, the library correctly falls back to creating a Job ID if the stateless query times out. This change updates the assertion logic to accept either a valid Query ID (stateless success) or a valid Job ID (fallback success).

Fixes #4008

* refactor: use XOR assertion for conciseness

 Applied feedback from code review to use exclusive OR operator for validating JobID/QueryID mutual exclusivity.

* fix: apply race condition logic to testStatelessQueries

Applied XOR assertion logic to testStatelessQueries. Test was failing on slow networks because they did not account for JOB_CREATION_OPTIONAL falling back to job creation.

Fixes #4002

* docs: add comment explaining stateless query fallback behavior

* docs: add comment explaining stateless query fallback behavior in testTableResultJobIdAndQueryId()

Author: SivamuruganP

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/it/ITBigQueryTest.java

@@ -7181,8 +7181,12 @@ public void testStatelessQueries() throws InterruptedException {
     // Stateless query should have no job id.
     bigQuery.getOptions().setDefaultJobCreationMode(JobCreationMode.JOB_CREATION_OPTIONAL);
     TableResult tableResult = executeSimpleQuery(bigQuery);
-    assertNotNull(tableResult.getQueryId());
-    assertNull(tableResult.getJobId());
+    // Use XOR: We accept EITHER a QueryId (fast path) OR a JobId (slow fallback), but not both.
+    // Ideally Stateless query will return queryId but in some cases it would return jobId instead
+    // of queryId based on the query complexity or other factors (job timeout configs).
+    assertTrue(
+        "Exactly one of jobId or queryId should be non-null",
+        (tableResult.getJobId() != null) ^ (tableResult.getQueryId() != null));
 
     // Job creation takes over, no query id is created.
     bigQuery.getOptions().setDefaultJobCreationMode(JobCreationMode.JOB_CREATION_REQUIRED);
@@ -7220,8 +7224,14 @@ public void testTableResultJobIdAndQueryId() throws InterruptedException {
     String query = "SELECT 1 as one";
     QueryJobConfiguration configStateless = QueryJobConfiguration.newBuilder(query).build();
     TableResult result = bigQuery.query(configStateless);
-    assertNull(result.getJobId());
-    assertNotNull(result.getQueryId());
+    // A stateless query should result in either a queryId (stateless success) or a jobId (fallback
+    // to a job).
+    // Exactly one of them should be non-null.
+    // Ideally Stateless query will return queryId but in some cases it would return jobId instead
+    // of queryId based on the query complexity or other factors (job timeout configs).
+    assertTrue(
+        "Exactly one of jobId or queryId should be non-null",
+        (result.getJobId() != null) ^ (result.getQueryId() != null));
 
     // Test scenario 2 by failing stateless check by setting job timeout.
     QueryJobConfiguration configQueryWithJob =