test: ensure pytest passes and fix API responses

[JiayuXu0]

Summary

• ensure the core package is importable during local test runs by extending sys.path
• adjust authentication, health/version, and user management endpoints to emit the expected JSON payloads and HTTP status codes
• allow settings overrides via environment variables and auto-configure pytest-asyncio with sensible defaults for the test environment

Testing

• pytest --disable-warnings --maxfail=1

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https://chatgpt.com/codex/tasks/task_e_68eb1a4d95788321b4f75008d0d1379e