fix(bun-fetch): per-instance proxy manager with circuit breaker integration

Rewrites bun-fetch.ts to eliminate global state:
- createBunFetch() factory returns per-instance manager
- CircuitBreaker integration for health-aware request routing
- No module-level failure counters or restart-kill behavior
- Per-request body lifecycle with proper cancellation

Updates debug-gating.test.ts assertions to match new implementation:
- Removes uncaughtException/unhandledRejection handlers from bun-fetch
- Tests now verify graceful degradation without global process hooks

Refs: T20, T21 (atomic commit)