fix: reject nested where clauses on missing fields

[LaplaceYoung]

Summary

• Treat nested _where predicates with missing or non-object intermediate fields as mismatches when they contain known operators.
• Preserve existing compatibility for unknown operator objects that do not contain known _where operators.
• Add regression coverage for missing nested fields, primitive intermediate fields, or clauses, and the GET /posts?_where=... request path.

Verification

• pnpm exec node --experimental-strip-types --test src/app.test.ts src/matches-where.test.ts
• pnpm test
• pnpm typecheck
• pnpm lint
• pnpm exec oxfmt --check src/app.test.ts src/matches-where.ts src/matches-where.test.ts
• pnpm exec tsc
• issue reproduction command now prints strict [] and bypass []
• git diff --check

pnpm build reaches the package script and fails on Windows because rm -rf lib is not available in PowerShell. pnpm exec tsc was used to verify the TypeScript build step directly.

Fixes #1731