Improve primitive route return guidance

[JacKane21]

Summary

• catches invalid primitive route returns before response serialization tries to inspect them
• reuses the existing ctx.json(...) guidance for raw object and primitive return mistakes
• adds regression coverage for raw string, number, boolean, and bigint returns while keeping valid Response/ctx.json(...) flows unchanged

/claim #30

Validation

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

Note

npm test -- tests/errors/do-not-allow-raw-json.test.ts currently stops in the package pretest build on pre-existing OpenAPI codegen TypeScript errors (operation.responses possibly undefined) before this test file runs.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve SerializableToResponse handling for non-WinterSpec types

Setting the fallback status to 200 here makes every non-Response/non-WinterSpecResponse return look successful, which sends custom SerializableToResponse objects into the validating path; serializeToResponse(...) then throws Unknown Response type for those objects because it only accepts WinterSpecResponse subclasses when validation is on. This is a regression for middleware/route handlers that return their own SerializableToResponse implementations (a supported type in Middleware/WinterSpecRouteFn) that previously serialized successfully.

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[JacKane21]

Closing this out as part of wrapping up my bounty/OSS cleanup. Thanks for the time and review.