fix(explore): send database backend as string in View Query format request

The View Query modal's SQL formatting toggle was returning a 400 Bad Request
error because the engine parameter was being sent as an object instead of a
string to the format_sql endpoint.

When the backend needs to be fetched from the dataset API, the code was
assigning the entire database object {backend: "postgresql"} rather than
extracting just the backend property string.

Changes:
- Use object destructuring to extract backend string from database object
- Add comprehensive test validating complete API payload structure
- Test verifies engine is sent as string, not object

Fixes #35682

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sadpandajoe

superset-frontend/src/explore/components/controls/ViewQuery.test.tsx

@@ -265,3 +265,29 @@ test('uses exploreBackend from Redux state when available', async () => {
   expect(formatCallBody.engine).toBe('postgresql');
   expect(fetchMock.calls(datasetApiEndpoint)).toHaveLength(0);
 });
+
+test('sends engine as string (not object) when fetched from dataset API', async () => {
+  const stateWithoutBackend = {
+    ...mockState(),
+    explore: undefined,
+  };
+
+  setup(mockProps, stateWithoutBackend);
+
+  await waitFor(() => {
+    expect(fetchMock.calls(datasetApiEndpoint)).toHaveLength(1);
+  });
+
+  await waitFor(() => {
+    expect(fetchMock.calls(formatSqlEndpoint)).toHaveLength(1);
+  });
+
+  const formatCallBody = JSON.parse(
+    fetchMock.lastCall(formatSqlEndpoint)?.[1]?.body as string,
+  );
+
+  expect(formatCallBody).toEqual({
+    sql: mockProps.sql,
+    engine: 'sqlite',
+  });
+});

superset-frontend/src/explore/components/controls/ViewQuery.tsx

@@ -106,7 +106,8 @@ const ViewQuery: FC<ViewQueryProps> = props => {
         const response = await SupersetClient.get({
           endpoint: `/api/v1/dataset/${datasetId}?q=${queryParams}`,
         });
-        backend = response.json.result.database;
+        const { backend: datasetBackend } = response.json.result.database;
+        backend = datasetBackend;
       }
 
       // Format the SQL query