feat(provider): define request queue contract

[luoye520ww]

Problem

Provider requests currently have no shared admission contract for per-provider/account concurrency and queue limits, making rate-limit storms and unbounded background work difficult to control consistently.

Scope

Adds a pure queue contract with provider/account keys, bounded concurrency and queue policies, explicit interactive/scheduled/background priorities, and fail-closed run/queue/reject admission decisions.

Non-goals

No provider client wiring, scheduler, circuit-breaker integration, persistence, or UI is included. Runtime queue execution is a separate follow-up PR.

Tests

• npm.cmd --prefix kun test -- tests/provider-request-queue.test.ts (9 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, cross-platform, compatibility, and scope review. No package smoke is claimed because this PR is a pure contract module.

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Superseded by consolidated provider/network safeguards PR #948, which passed Typecheck, Linux, macOS, and Windows packaged checks.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.