test: cover review scheduling and WeChat reply failures

[Kingson4Wu]

Summary

• Add deterministic review schedule helper coverage for due ordering, upcoming windows, minimum bounds, date validation, and invalid integer fallback.
• Add WeChat channel regression tests for missing sender/self-message rejection and blocked-message reply delivery failure logging without invoking the agent.
• Refresh generated coverage badge from 85.45% to 85.48%.

Verification

• pnpm vitest run tests/unit/review-schedule.test.ts tests/integration/wechat-channel.test.ts
• pnpm run lint
• npm run typecheck
• npm test
• pnpm run test:coverage
• loop architecture assessment: score 98/95
• pre-push project-health passed via git push hook

Notes

• No production code changes.
• Coverage after: statements 82.70%, branches 69.16%, functions 86.45%, lines 85.48%.