test: cover LLM quota guard

[beanscg]

Summary

Adds focused Vitest coverage for server/utils/llm-quota-guard.ts, one of the smaller uncovered server/utils helpers called out by the coverage work in #68.

The new tests cover:

• suppression after provider quota errors
• app rate-limit errors not being treated as provider quota suppression
• explicit/manual quota-hit marking with no error argument
• cooldown expiry after five minutes

This keeps the first slice narrow and does not raise coverage thresholds.

Validation

The local shell does not have a global pnpm shim, so I ran the same project commands through npx --package pnpm@10.12.4 pnpm ... using Node 24.14.0.

• pnpm install --frozen-lockfile
• pnpm test -- tests/llm-quota-guard.spec.ts
• pnpm lint
• pnpm typecheck
• pnpm test:coverage
• SKIP_ENV_VALIDATION=true pnpm build
• pnpm ci:check
• git diff --check HEAD~1 HEAD

Coverage report showed server/utils/llm-quota-guard.ts at 100% statements/branches/functions/lines, and the full project thresholds stayed green.

Submitted by Bean Labs as a public free-service contribution; acceptance is still pending maintainer review.