fix: reject symbol route returns

[Qian001A]

/claim #30

Summary

• reject Symbol values returned directly from route handlers with ctx.json guidance
• keep response validation error formatting stable when it wraps a non-Zod Error
• add a focused regression test for the symbol-return path

Why

Returning a Symbol currently falls through the serializer and produces a low-level JavaScript error from the JavaScript in operator. This gives users a direct, actionable message for that unsupported route return value.

Validation

• npx ava tests/errors/do-not-allow-raw-json.test.ts --verbose
• npx prettier --check src/create-with-winter-spec.ts src/lib/format-zod-error.ts tests/errors/do-not-allow-raw-json.test.ts
• git diff --check