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_api_contracts.py

@@ -160,7 +160,9 @@ if SCHEMATHESIS_AVAILABLE:
 
         def test_schema_has_user_operations(self):
             """Verify user-related operations exist."""
-            user_ops = list(schema.include(path_regex=r".*users.*").get_all_operations())
+            user_ops = list(
+                schema.include(path_regex=r".*users.*").get_all_operations()
+            )
             assert len(user_ops) > 0, "No user operations found"
 
         def test_schema_has_organization_operations(self):
@@ -172,7 +174,9 @@ if SCHEMATHESIS_AVAILABLE:
 
         def test_schema_has_admin_operations(self):
             """Verify admin-related operations exist."""
-            admin_ops = list(schema.include(path_regex=r".*admin.*").get_all_operations())
+            admin_ops = list(
+                schema.include(path_regex=r".*admin.*").get_all_operations()
+            )
             assert len(admin_ops) > 0, "No admin operations found"
 
         def test_schema_has_session_operations(self):