Handle undefined route returns with helpful guidance

[CharlieLZ]

Summary

• add a regression test for route handlers that accidentally return undefined
• guard missing/primitive route responses before response serialization reads .status
• surface an actionable message telling users to use ctx.json({...}) or return a Response

Why

Current main still allows undefined route returns to reach serializeResponse, where .status is read before the existing raw-object guidance middleware can run. That produces a low-level serialization failure instead of the issue #30 guidance.

This is narrower than the raw-object case and keeps existing return { ... } behavior intact.

Verification

• npm test -- tests/errors/do-not-allow-raw-json.test.ts -> 2 passed
• npm run lint -> passed
• npm run typecheck -> passed
• npm test -> 186 passed, 18 skipped

/claim #30

[CharlieLZ]

Ready for review. This PR adds explicit guidance for route handlers that resolve to undefined, with focused coverage for the regression case.\n\nVerification run locally:\n- npm test -- tests/errors/do-not-allow-raw-json.test.ts\n- npm run lint\n- npm run typecheck\n- npm test\n\nHappy to adjust wording or error shape if you prefer a different developer-facing message.