feat(provider): define circuit breaker policy

[luoye520ww]

Problem

Provider failures currently lack a shared, deterministic circuit-breaker policy, so runtime integration cannot safely distinguish retryable outages from authentication and client errors.

Scope

This PR adds the provider circuit-breaker state machine only. It handles closed/open/half-open transitions, one half-open probe, bounded failure thresholds, and fail-closed validation for untrusted state, policy, failure kinds, and timestamps.

Non-goals

Provider queue, request execution, UI, and runtime wiring are intentionally separate follow-up PRs.

Tests

• npm.cmd --prefix kun test -- src/services/provider-circuit-breaker.test.ts (6 passed)
• npm.cmd run typecheck
• npm.cmd --prefix kun run typecheck
• npm.cmd run lint
• npm.cmd run build
• git diff --check

Review

Completed functional, concurrency/lifecycle, data integrity, security, compatibility, and scope review. No package smoke is claimed because this PR contains a pure runtime policy module without an entrypoint change.

Issue

Part of #885

[luoye520ww]

Superseded by consolidated provider/network safeguards PR #948, which passed Typecheck, Linux, macOS, and Windows packaged checks. Closing this duplicate to keep the review surface focused.