Improve raw object return error to preserve ctx.json guidance

[liangtovi-debug]

Fixes #30\n\n### What changed\n- Preserve non-Zod error messages in \n- Add regression test to ensure raw-object route returns keep the guidance message\n\n### Verification\n-

winterspec@0.0.114 pretest
npm run build

winterspec@0.0.114 prebuild
del dist

winterspec@0.0.114 build
tsc -p tsconfig.build.json && tsc-alias -p tsconfig.build.json

winterspec@0.0.114 test
ava tests/errors/do-not-allow-raw-json.test.ts tests/errors/raw-json-preserves-helpful-message.test.ts

✔ do-not-allow-raw-json › should throw an error when responding with raw JSON
✔ raw-json-preserves-helpful-message › returning raw object keeps ctx.json guidance message
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2 tests passed\n