Add endpoint health management: circuit breaker and background probing

[N-thnI]

Description

Extend RPC endpoint health handling with a proper breaker and proactive recovery.

Problem Statement

RpcClient currently quarantines an endpoint for a fixed duration after N transport failures — deliberately simple, but it neither backs off progressively for an endpoint failing repeatedly, nor probes carefully before restoring full traffic. Recovery is also only discovered when a user request happens to retry a quarantined endpoint, so the first request after an outage pays the latency cost. Both concerns touch the same health state, which is why they belong in one change.

Proposed Changes

• src/resilience/circuit-breaker.ts with closed/open/half-open states; in half-open allow a single probe before fully restoring; expose state transitions for logging
• src/resilience/health-scheduler.ts probing quarantined endpoints on a configurable interval, with start()/stop() where stop() clears every timer — no dangling handles keeping a process alive
• Both opt-in: RpcClient must behave exactly as it does today when neither is supplied
• Tests for both

Technical Implementation Scaffolding

• Target Repository: vero-sdk
• Target Path: src/resilience/
• Branch Naming: feat/issue--endpoint-health
• Authority Context: Reliability

Acceptance Criteria

• A failing half-open probe reopens the breaker without restoring full traffic
• A recovered endpoint returns to rotation without a user request triggering it
• stop() leaves no active timers
• Existing RpcClient behaviour is unchanged without opt-in
• npm test, npm run typecheck, npm run lint, and npm run build all pass

Definition of Done

• Reviewed by lead maintainer
• Pull request merged via verified status check

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This issue is self-contained. Everything it needs already exists on main; it does not wait on any other issue. Deliver the module and its tests in one PR.

[Adeyemi-cmd]

Hello maintainer. I would love to work on this

[grantfox-oss]

🦊 GrantFox — @Adeyemi-cmd has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #49)
2. Your PR will be reviewed by the Vero-protocol maintainers

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