test(bigquery): Fix flaky ITBigQueryTest#testProjectIDFastSQLQueryWithJobId

This PR fixes a flaky integration test by replacing a strict string assertion
on a backend error message with a more robust assertion on the HTTP status
code (400 Bad Request).

It also refactors the test to use JUnit's \`assertThrows\` for cleaner and
more idiomatic exception testing, automatically preventing false positives
if the exception isn't thrown.

Fixes #12135

Author: jinseopkim0

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

@@ -4698,13 +4698,10 @@ void testProjectIDFastSQLQueryWithJobId() {
     JobId jobIdWithProjectId = JobId.newBuilder().setProject(invalidProjectId).build();
     QueryJobConfiguration configSelect =
         QueryJobConfiguration.newBuilder(query).setDefaultDataset(DatasetId.of(DATASET)).build();
-    try {
-      bigquery.query(configSelect, jobIdWithProjectId);
-    } catch (Exception exception) {
-      // error message for non-existent project
-      assertEquals("Cannot parse  as CloudRegion.", exception.getMessage());
-      assertEquals(BigQueryException.class, exception.getClass());
-    }
+    BigQueryException bqException =
+        assertThrows(
+            BigQueryException.class, () -> bigquery.query(configSelect, jobIdWithProjectId));
+    assertEquals(400, bqException.getCode());
   }
 
   @Test