fix(catalog): prevent prototype pollution in path utilities

[jong-k]

Summary

setByPath and getByPath in @cloudflare/kumo/catalog walk untrusted, AI-generated JSON Pointer paths against the data model. Both traversed the prototype chain, so a path like /__proto__/polluted or /constructor/prototype/polluted could write to Object.prototype, and /constructor or /toString could read prototype members into resolved props / visibility checks.

• Reject __proto__, constructor, and prototype segments in both getByPath and setByPath (defense-in-depth).
• Use Object.hasOwn instead of in / bare indexing so only own properties are traversed.
• Treat null intermediates the same as missing (previously typeof null === "object" let traversal fall through and throw).
• Skip writes to an array's length (previously RangeError: Invalid array length on /items/length/x).
• Return early for empty / root paths in setByPath, mirroring getByPath.
• Add regression tests and a patch changeset.

No behavior change for plain JSON data models (DataModel = Record<string, unknown>); array index access (/items/0) still works.

Testing

• pnpm --filter @cloudflare/kumo test: 59 files, 1403 passed
• pnpm --filter @cloudflare/kumo lint
• pnpm --filter @cloudflare/kumo typecheck
• pnpm vp fmt --check on changed files
• git diff --check

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• Reviews
  • bonk has reviewed the change
  • automated review not possible because: external contributor, no access to bonk
• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because: