fix: include method/path context in plain-object route return error

[tungnguyentu]

Summary

When a route handler returns a plain object instead of ctx.json(), the error message now includes the HTTP method and path so developers can immediately identify which handler needs fixing.

Before:

Use ctx.json({...}) instead of returning an object directly.

After:

Route handler for GET /things/list returned a plain object. Use ctx.json({...}) instead of returning an object directly.
Example fix: return ctx.json({ ok: true })

This makes the error actionable — you can see exactly which route has the bug and what to do to fix it.

/claim #30