fix: release half-open trial slots after breaker transitions

[Bashman10]

Overview

This PR fixes a state-leak bug in CircuitBreaker.execute where the activeTrials entry for a breaker was only removed in finally when metrics.state was still HALF_OPEN. Once a trial call closed or re-opened the breaker, the key was left behind, so the next HALF_OPEN window rejected every call with "Only one trial call allowed" — permanently wedging the breaker until restart.

The fix tracks slot acquisition in a local flag and always releases the slot in finally when this invocation actually acquired it. A regression test cycles the breaker through two full OPEN → HALF_OPEN → CLOSED recoveries under fake timers.

Related Issue

Changes

🔧 Circuit breaker fix

• [MODIFY] src/lib/circuitBreaker.ts
  • In execute, introduce a local acquiredTrialSlot flag set only when this invocation successfully adds breakerKey to activeTrials in the HALF_OPEN branch.
  • In finally, delete breakerKey from activeTrials whenever acquiredTrialSlot is true, regardless of the resulting metrics.state (previously gated on state === HALF_OPEN).
  • Preserves the existing single-trial guard: concurrent callers that fail to acquire the slot still throw "Only one trial call allowed" and never touch activeTrials.

🧪 Regression tests

• [MODIFY] src/lib/circuitBreaker.test.ts
  • Adds a fake-timer regression that drives the breaker through two consecutive recovery cycles (OPEN → HALF_OPEN → CLOSED → OPEN → HALF_OPEN → CLOSED) and asserts both cycles succeed.
  • Asserts activeTrials is empty after each execute call completes (success, failure, and rejection paths).
  • Asserts concurrent calls during HALF_OPEN still allow exactly one trial and reject the rest.

Verification Results

npm test -- src/lib/circuitBreaker.test.ts src/routes/gatewayRoutes.circuitBreaker.test.ts
✅ circuitBreaker.test.ts passed (including the new two-cycle regression)
✅ gatewayRoutes.circuitBreaker.test.ts passed

Acceptance Criteria	Status
A breaker completes two full recovery cycles in a fake-timer test	✅ Regression test drives OPEN → HALF_OPEN → CLOSED twice under fake timers
activeTrials is empty after any execute call completes	✅ Asserted after success, failure, and rejected-trial paths
Concurrent calls in HALF_OPEN still allow only one trial	✅ Existing guard retained; concurrency assertion added
src/lib/circuitBreaker.test.ts contains the regression	✅ New test added in that file

Security and Failure-Mode Handling

• No safeguard weakening: the HALF_OPEN single-trial admission check is unchanged; only the release path is corrected. Callers that do not acquire the slot still fail fast.
• Failure-mode safety: releasing the slot in finally on the acquiring invocation guarantees the slot is freed whether the trial succeeds (→ CLOSED), fails (→ OPEN), or throws unexpectedly. This prevents a transient upstream outage from becoming permanent.
• No new external surface: change is confined to internal bookkeeping in circuitBreaker.ts; no API, config, or dependency changes.

Compatibility

No public API or behavioral contract changes beyond the bug fix. Existing callers see identical semantics for the single-trial guard; the only observable difference is that breakers can now recover across multiple cycles instead of latching after the first.

Closes #1270