fix: reject function route returns

[Qian001A]

/claim #30

Summary

• reject function values returned directly from route handlers
• add a regression test that confirms the invalid return gets the existing ctx.json guidance

Why

Issue #30 asks for clearer guidance when handlers return unsupported values directly instead of using ctx.json. Existing open PRs cover object/null/primitive cases; this covers the function-return gap without changing the existing message contract.

Validation

• npx ava tests/errors/do-not-allow-raw-json.test.ts --verbose
• npx prettier --check src/middleware/with-response-object-check.ts tests/errors/do-not-allow-raw-json.test.ts
• git diff --check