Improve invalid route return guidance

[zergzorg]

Summary

• keeps the existing raw object guidance for accidental return { ... } route handlers
• adds early guards for null, undefined, primitive, symbol, and function route returns so they produce the actionable ctx.json(...) guidance before response serialization fails
• preserves valid custom SerializableToResponse objects so supported response wrappers still pass through
• adds regression coverage for raw object, null, undefined, primitive, symbol, function, and custom serializable-response paths

/claim #30

Validation

• npm run build
• npm test -- tests/errors/do-not-allow-raw-json.test.ts
• npm run lint -- --ignore-path .gitignore
• npm run typecheck
• git diff --check

Notes

This is a narrow diagnostic follow-up for #30. It does not change valid Response, ctx.json(...), or custom serializable response return paths.

[zergzorg]

Follow-up after checking fresh #30 attempts:

• Added the same actionable guidance path for raw primitive route returns (string, number, boolean, bigint), not only object/null/undefined returns.
• The guard now rejects anything that is not a Response or WinterSpec serializable response before serialization tries to inspect it.
• Added focused AVA coverage for the primitive return cases.

Validation:

• npx ava tests/errors/do-not-allow-raw-json.test.ts
• npm run build
• npx prettier --check src/middleware/with-response-object-check.ts tests/errors/do-not-allow-raw-json.test.ts
• git diff --check

[zergzorg]

Follow-up after the latest #30 attempts:

• Added regression coverage for raw symbol and raw function route returns.
• This uses the existing validation path from this PR and keeps the scope to actionable guidance for non-Response handler returns.

Validation: npm test -- tests/errors/do-not-allow-raw-json.test.ts

[zergzorg]

Follow-up regression coverage pushed in 8cd0b85:

• added a test that custom SerializableToResponse objects still pass through the raw-return guard
• keeps the invalid raw primitive/object/null coverage while protecting the supported custom response path

Validation:

• npm run build
• npm test -- tests/errors/do-not-allow-raw-json.test.ts
• git diff --check
• GitHub build is passing on the new head

[zergzorg]

Follow-up clarification pushed in d75113e after re-checking newer same-issue guidance PRs:

• the invalid route-return error now recommends ctx.json(value) instead of implying only object literals are valid JSON responses
• the message now covers raw primitive/null/undefined returns and still points users toward Response-compatible returns
• tests now assert against the shared error-message constant so future wording changes stay aligned with the middleware

Validation:

• npm test -- tests/errors/do-not-allow-raw-json.test.ts
• npm run build
• npm run lint -- src/middleware/with-response-object-check.ts tests/errors/do-not-allow-raw-json.test.ts
• git diff --check