Add guidance for invalid route return values

[momenbuilds]

Summary

• Add an early invalid-return guard for route handlers that return null, primitives, or functions.
• Surface actionable guidance to use ctx.json({...}) or return a Response instead of letting low-level response serialization fail.
• Add regression coverage for undefined, null, string, number, boolean, symbol, bigint, and function returns while preserving the existing raw object test.

Validation

• npm test -- tests/errors/do-not-allow-raw-json.test.ts
• npm run lint -- src/create-with-winter-spec.ts tests/errors/do-not-allow-raw-json.test.ts
• git diff --check

/claim #30

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR strengthens response serialization by explicitly rejecting primitive return values from route handlers and adds tests to ensure users get an actionable error message.

Changes:

• Add a runtime guard in response serialization to throw a clearer error when a handler returns a primitive (or null) directly.
• Add parameterized tests covering several invalid primitive return types (e.g., undefined, string, number, symbol, bigint).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/errors/do-not-allow-raw-json.test.ts	Adds a table-driven test suite to verify actionable errors for primitive handler returns.
src/create-with-winter-spec.ts	Adds a primitive/null guard in serializeResponse with an improved error message.

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[momenbuilds]

Follow-up pushed in 809c6b3 addressing automated review feedback: the primitive-return guidance now says ctx.json(value) instead of implying only object literals, and the test middleware now handles non-Error throws safely. Local validation: npm test -- tests/errors/do-not-allow-raw-json.test.ts, npm run lint -- src/create-with-winter-spec.ts tests/errors/do-not-allow-raw-json.test.ts, git diff --check. CI is green again.