follow-up: Improve test isolation by resetting shared context variables between tests for test_gitlab_workflow.py

Problem

Reference comment - !3372 (comment 2784735948)

Currently, tests use shared context variables (like current_user) that can leak state between test runs, potentially causing flaky test behavior in CI pipelines.

Example from tests/duo_workflow_service/checkpointer/test_gitlab_workflow.py:

current_user.set(mock_user)

Impact

• Risk of flaky test assertions due to state leakage
• Tests may pass/fail inconsistently depending on execution order
• Reduced reliability of CI pipeline

Proposed Solution

Ensure all shared context variables are properly reset after each test. This could involve:

1. Teardown methods: Add cleanup in test teardown to reset context variables
2. Test fixtures: Create fixtures that automatically clean up context after each test
3. Explicit mocking: Use more isolated mocking strategies per test

Scope

This affects multiple test files across the codebase where shared context variables are used. A codebase-wide audit should identify all instances of this pattern.