fix(bigquery): restore IF NOT EXISTS in create_search_index and clean up test mocks

Problem:
Following review feedback across the BigQuery sample PRs (#14714, #14715,
#14717):
- `bigquery_create_search_index.py` should use `CREATE SEARCH INDEX IF NOT EXISTS`
  and handle `exceptions.NotFound` when the parent table or dataset is missing.
- `bigquery_drop_vector_index.py` `NotFound` error message should reflect that
  `DROP VECTOR INDEX IF EXISTS` raises `NotFound` when the parent table or
  dataset is missing.
- Unit tests in `bigquery_create_search_index_test.py`,
  `bigquery_drop_vector_index_test.py`, and
  `bigquery_query_manifest_external_table_test.py` defined unused mock
  attributes (`mock_row` on DDL statements and `mock_job.statement_type` on
  SELECT queries).

Solution:
- Restore `CREATE SEARCH INDEX IF NOT EXISTS` and catch `exceptions.NotFound`
  in `bigquery/search/bigquery_create_search_index.py` and its unit test.
- Update the `NotFound` error message in
  `bigquery/vector_search/bigquery_drop_vector_index.py`.
- Remove unused `mock_row` and `mock_job.statement_type` setup in the three
  corresponding unit test files.

Author: iennae

bigquery/biglake/bigquery_query_manifest_external_table_test.py

@@ -26,7 +26,6 @@ def test_query_manifest_external_table_success(
     mock_client = mock.MagicMock()
     mock_job = mock.MagicMock()
     mock_job.job_id = "example-job-123"
-    mock_job.statement_type = "SCRIPT"
     mock_row = mock.MagicMock()
     mock_job.result.return_value = [mock_row]
     mock_client.query.return_value = mock_job
@@ -54,7 +53,9 @@ def test_query_manifest_external_table_error(
     capsys: pytest.CaptureFixture[str],
 ) -> None:
     mock_client = mock.MagicMock()
-    mock_client.query.side_effect = exceptions.NotFound("resource error")
+    mock_job = mock.MagicMock()
+    mock_job.result.side_effect = exceptions.NotFound("resource error")
+    mock_client.query.return_value = mock_job
     monkeypatch.setattr(
         bigquery_query_manifest_external_table.bigquery,
         "Client",

bigquery/search/bigquery_create_search_index.py

@@ -38,18 +38,18 @@ def create_search_index(
     table_ref = f"{project_id}.{dataset_id}.{table_id}"
     # Indexing ALL COLUMNS enables full-text SEARCH queries across every STRING or JSON field.
     sql = f"""
-        CREATE SEARCH INDEX `{index_name}`
+        CREATE SEARCH INDEX IF NOT EXISTS `{index_name}`
         ON `{table_ref}`(ALL COLUMNS);
     """
 
     try:
         print(f"Creating search index {index_name} on table: {table_ref}")
         query_job = client.query(sql)
         query_job.result()
-    except exceptions.AlreadyExists as err:
+    except exceptions.NotFound as err:
         print(
-            f"error: search index '{index_name}' already exists on table "
-            f"'{table_id}' in dataset '{dataset_id}' for project '{project_id}': {err.message}",
+            f"error: table '{table_id}' or dataset '{dataset_id}' not found "
+            f"for project '{project_id}': {err.message}",
             file=sys.stderr,
         )
         return

bigquery/search/bigquery_create_search_index_test.py

@@ -27,8 +27,7 @@ def test_create_search_index_success(
     mock_job = mock.MagicMock()
     mock_job.job_id = "example-job-123"
     mock_job.statement_type = "SCRIPT"
-    mock_row = mock.MagicMock()
-    mock_job.result.return_value = [mock_row]
+    mock_job.result.return_value = []
     mock_client.query.return_value = mock_job
     monkeypatch.setattr(
         bigquery_create_search_index.bigquery,
@@ -53,7 +52,9 @@ def test_create_search_index_error(
     capsys: pytest.CaptureFixture[str],
 ) -> None:
     mock_client = mock.MagicMock()
-    mock_client.query.side_effect = exceptions.AlreadyExists("resource error")
+    mock_job = mock.MagicMock()
+    mock_job.result.side_effect = exceptions.NotFound("resource error")
+    mock_client.query.return_value = mock_job
     monkeypatch.setattr(
         bigquery_create_search_index.bigquery,
         "Client",

bigquery/vector_search/bigquery_drop_vector_index.py

@@ -45,8 +45,8 @@ def drop_vector_index(
         query_job.result()
     except exceptions.NotFound as err:
         print(
-            f"error: table '{table_id}' or vector index '{index_name}' not found in "
-            f"dataset '{dataset_id}' for project '{project_id}': {err.message}",
+            f"error: table '{table_id}' or dataset '{dataset_id}' not found in "
+            f"project '{project_id}': {err.message}",
             file=sys.stderr,
         )
         return

bigquery/vector_search/bigquery_drop_vector_index_test.py

@@ -27,8 +27,7 @@ def test_drop_vector_index_success(
     mock_job = mock.MagicMock()
     mock_job.job_id = "example-job-123"
     mock_job.statement_type = "SCRIPT"
-    mock_row = mock.MagicMock()
-    mock_job.result.return_value = [mock_row]
+    mock_job.result.return_value = []
     mock_client.query.return_value = mock_job
     monkeypatch.setattr(
         bigquery_drop_vector_index.bigquery,
@@ -53,7 +52,9 @@ def test_drop_vector_index_error(
     capsys: pytest.CaptureFixture[str],
 ) -> None:
     mock_client = mock.MagicMock()
-    mock_client.query.side_effect = exceptions.NotFound("resource error")
+    mock_job = mock.MagicMock()
+    mock_job.result.side_effect = exceptions.NotFound("resource error")
+    mock_client.query.return_value = mock_job
     monkeypatch.setattr(
         bigquery_drop_vector_index.bigquery,
         "Client",