test: cover malformed query cache fallback

[attaboy11]

Summary

• Treat a fresh cache row with malformed JSON as a cache miss instead of throwing out of beforeQueryCache.
• Log the parse failure so corrupt cache rows remain diagnosable.
• Add focused Vitest coverage for the malformed cached-result path in src/cache/index.test.ts.

/claim #71

Validation

• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec vitest run src/cache/index.test.ts -> 15 tests passed
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec vitest run src/cache/index.test.ts --coverage.enabled true --coverage.include src/cache/index.ts --coverage.reporter text --coverage.thresholds.lines=0 --coverage.thresholds.branches=0 --coverage.thresholds.functions=0 --coverage.thresholds.statements=0 -> passed; src/cache/index.ts at 89.79% statements / 81.39% branches / 100% functions / 88.88% lines
• COREPACK_ENABLE_PROJECT_SPEC=0 corepack pnpm exec prettier --check src/cache/index.ts src/cache/index.test.ts -> passed
• git diff --check -> passed

Notes

• Full suite still has the existing unrelated RLS failures in src/rls/index.test.ts (4 failing assertions); src/cache/index.test.ts passes within that run.
• No UI demo video is included because this is a backend/unit-test-only cache coverage change with no visible UI surface.