Throw clear error when route handler returns plain object

[gavin-openops]

/claim #30

Summary

• improve the raw-object response error message to clearly instruct using ctx.json(...)
• add a test that verifies the clear ctx.json(...) guidance is logged when a route returns a plain object
• keep existing error-capture test and assert on the updated message

Testing

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