test: cover cache bypass and fallback paths

[Mykidzou]

/claim #71

Summary

• Focused incremental coverage for cache behavior.
• Adds Vitest coverage for cache bypass paths before and after query execution.
• Covers cache miss lookup behavior, default TTL inserts, and cache write error logging/fallback behavior.
• Test-only coverage change: no runtime behavior changes, no security research, no live probing.

Validation

• CI=true pnpm test src/cache/index.test.ts - passed, 20 tests
• pnpm exec vitest run src/cache/index.test.ts --coverage.enabled true --coverage.include src/cache/index.ts --coverage.reporter text - passed; src/cache/index.ts coverage is 95.65% statements, 88.37% branches, 100% functions, 95.23% lines
• pnpm exec prettier --check src/cache/index.test.ts - passed
• pnpm run build - passed
• git diff --check - passed
• pnpm lint - unavailable; no lint script is defined in package.json
• pnpm typecheck - unavailable; no typecheck script is defined in package.json

No active open PR besides this one touches src/cache/index.test.ts; #139, #149, and #150 cover allowlist, CSV import, and RLS respectively.

No demo video included because this is backend/unit-test-only coverage with no UI or runtime behavior change.