test(analytics): add comprehensive integration and edge-case tests

Express integration tests (supertest): middleware tracks real requests,
handles concurrent load, skips health routes, captures query strings.

Service resilience tests: SDK error propagation (sync + async),
post-shutdown no-op safety, double-shutdown idempotency, optional args.

Init wiring tests: module init calls AnalyticsService.init(), registers
middleware, sets up billing plan.changed listener with error isolation.

Feature-flag middleware edge cases: falsy _id variants (null, 0, empty),
ObjectId-like coercion, concurrent flag evaluations, 403 response shape.

Closes #3298

Author: PierreBrisorgueil