feat: add advanced middleware unit tests (#424)

[Ummi-001]

What I did

• Created tests/unit/advanced/ folder
• Added circuit-breaker.spec.ts — tests all 3 state transitions using jest.useFakeTimers()
• Added async-context.spec.ts — tests concurrent context isolation with no leakage
• Added webhook-signature.spec.ts — tests Stripe, GitHub, Slack + replay attack
• Added adaptive-rate-limiter.spec.ts — mocks os.cpus() for high/low load simulation.

Closes #423
Closes #424
Closes #425
Closes #426