fix: throw clear error for invalid route return values

[MikeGarciaAGM]

Closes #30

Summary

• Throw a clear guidance error when a route/middleware returns undefined, null, or primitive values.
• Keep existing explicit guidance for raw object returns: use ctx.json({...}).
• Add regression test covering undefined route returns.

Why

This avoids confusing low-level failures and points users directly to the correct response API.

Validation

• npm test -- tests/errors/do-not-allow-raw-json.test.ts

/claim #30

[MikeGarciaAGM]

Follow-up: this PR now includes /claim #30 and is ready for payout review. Validation already run locally: npm test -- tests/errors/do-not-allow-raw-json.test.ts. Happy to adjust quickly if maintainers want narrower behavior.