Handle invalid route returns with ctx.json guidance

[Squirbie]

Summary

• add an early route-response guard for unsupported handler return values before response serialization reads status or uses the in operator
• keep the existing raw-object guidance message and add AVA coverage for undefined, null, primitive, symbol, and function returns

/claim #30

Validation

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

Notes

• npm ci is currently blocked because package-lock.json is not in sync with package.json for esbuild optional packages.
• npm test -- tests/errors/do-not-allow-raw-json.test.ts reaches the pretest build and is blocked by existing TypeScript errors in src/cli/commands/codegen/openapi.ts and src/cli2/commands/codegen/openapi.ts about operation.responses possibly being undefined. The focused AVA regression test above passes.

[Squirbie]

Follow-up after checking the open PR queue: this overlaps materially with earlier clean invalid-route-return PRs, especially #156 and #157, and partially with narrower cases like #158/#152/#154. I missed those before opening this PR. Please treat this PR as an alternate/reference implementation rather than a first claim; I am happy to close it if maintainers prefer to keep the earlier candidates active.