feat(observability): add circuit-breaker state transition metrics

[nobodieair]

Overview

This PR adds observability for circuit-breaker state transitions. The breaker now emits counters whenever it transitions to OPEN, HALF_OPEN, or CLOSED, using the existing metrics registry. This makes breaker open/half-open/close events visible in dashboards and alerts without changing the breaker’s behavior.

Related Issue

Changes

🔁 Circuit-Breaker Transition Metrics

• [MODIFY] src/utils/circuitBreaker.ts

  • Emit a metric on every state transition:
    • circuit_breaker_opened_total when transitioning to OPEN
    • circuit_breaker_half_opened_total when transitioning to HALF_OPEN
    • circuit_breaker_closed_total when transitioning to CLOSED
  • Emit exactly once per transition; repeated isOpen() / state checks do not double-count.
  • Preserve existing behavior while reporting transition metadata to the metrics layer.

• [MODIFY] src/lib/monitoring/metrics.ts

  • Add counter definitions for open, half-open, and closed transitions.
  • Export recordCircuitBreakerTransition(previousState, nextState) to centralize metric emission.
  • Follow existing naming and registry conventions.

• [ADD] src/utils/__tests__/circuitBreaker.test.ts and src/lib/__tests__/circuitBreakerMetrics.test.ts

  • Verify each state transition increments the correct counter once.
  • Verify no-op transitions and repeated state checks do not produce duplicates.
  • Verify existing circuit-breaker tests still pass.

Verification Results

npm test -- src/utils/__tests__/circuitBreaker.test.ts src/lib/__tests__/circuitBreakerMetrics.test.ts
✅ 16/16 passed

Manual verification:
✅ Open transition emitted exactly once
✅ Half-open transition emitted exactly once
✅ Closed transition emitted exactly once
✅ Repeated state checks produce no extra metrics

Acceptance Criteria	Status
Implemented across the listed files	✅ circuitBreaker.ts and metrics.ts updated
Unit/integration tests added or updated and passing	✅ 16/16 tests pass
No regression; follows project coding standards	✅ Metrics are additive; breaker state-machine behavior unchanged

Closes #1165

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