fix: reject empty JSON-RPC batches

[vsolano9]

Closes #4

What & why

JSON-RPC 2.0 requires an empty batch to produce one Invalid Request response object. Casebook instead fell through to its response-free batch path and returned HTTP 202.

The guard now rejects [] before dispatch with code -32600 and id: null, matching the official batch specification. Non-empty notification-only batches remain response-free.

Changes

• reject empty arrays before batch dispatch
• cover the exact error payload at the HTTP request boundary
• preserve and cover the notification-only 202 behavior

Validation

• npm test — 14/14 passed
• npm run typecheck — passed
• npx wrangler deploy --dry-run — worker bundle succeeded, no deployment
• focused batch suite — 20 repeated runs passed

The repository defines no format or lint scripts; git diff --check passes and the two changed files follow the existing style.

Checklist

• Focused change with a clear description
• Ran all available validation
• Docs or CHANGELOG update is not needed for this protocol compliance fix
• Linked the related issue

AI assistance: Codex helped implement and validate the patch; the final diff and test output were reviewed before submission.