Refactor tests for improved readability and fixture consistency

- Reformatted headers in E2E tests to improve readability and ensure consistent style.
- Updated confidential client fixture to use bcrypt for secret hashing, enhancing security and testing backward compatibility with legacy SHA-256 hashes.
- Added new test cases for PKCE verification, rejecting insecure 'plain' methods, and improved error handling.
- Refined session workflows and user agent handling in E2E tests for session management.
- Consolidated schema operation tests and fixed minor formatting inconsistencies.

backend/tests/e2e/test_admin_superuser_workflows.py

@@ -429,6 +429,7 @@ class TestAdminSessionManagement:
         data = response.json()
         assert "data" in data
 
+
 class TestAdminDeleteOperations:
     """Test admin delete operations."""
 
@@ -645,5 +646,3 @@ class TestAdminSearchAndFilter:
         assert response.status_code == 200
         data = response.json()
         assert len(data["data"]) >= 1
-
-