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_user_workflows.py

@@ -21,7 +21,7 @@ pytestmark = [
 ]
 
 
-async def register_and_login(client, email: str, password: str = "SecurePassword123!"):
+async def register_and_login(client, email: str, password: str = "SecurePassword123!"):  # noqa: S107
     """Helper to register a user and get tokens."""
     await client.post(
         "/api/v1/auth/register",