test(bigquery): Fix flaky ITBigQueryTest#testProjectIDFastSQLQueryWithJobId

This PR fixes a flaky integration test by replacing a strict string assertion
on a backend error message ("Cannot parse as CloudRegion.") with a more robust
assertion on the HTTP status code (400 Bad Request) and a flexible substring
check.

Fixes #12135

Author: jinseopkim0

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

@@ -4700,10 +4700,12 @@ void testProjectIDFastSQLQueryWithJobId() {
         QueryJobConfiguration.newBuilder(query).setDefaultDataset(DatasetId.of(DATASET)).build();
     try {
       bigquery.query(configSelect, jobIdWithProjectId);
+      fail("Expected BigQueryException to be thrown");
     } catch (Exception exception) {
-      // error message for non-existent project
-      assertEquals("Cannot parse  as CloudRegion.", exception.getMessage());
       assertEquals(BigQueryException.class, exception.getClass());
+      BigQueryException bqException = (BigQueryException) exception;
+      assertEquals(400, bqException.getCode());
+      assertTrue(bqException.getMessage().contains("CloudRegion"));
     }
   }