fix(middleware): propagate return plain object error message instead of masking (#30)

[Heyzerohey]

Closes #30.

This PR fixes the issue where returning a plain object return { ... } from a route instead of return ctx.json({ ... }) was masked as a generic 500 Internal server error in production/default exception handling.

Changes

1. Defined ResponseObjectNotAllowedError: Added a subclass of WinterSpecMiddlewareError in src/middleware/http-exceptions.ts containing the helpful error message "Use ctx.json({...}) instead of returning an object directly." with a status of 500.
2. Updated withResponseObjectCheck: Throw ResponseObjectNotAllowedError in src/middleware/with-response-object-check.ts instead of throwing a generic Error.
3. Added Regression Test: Implemented a comprehensive test case in tests/middleware/with-response-object-check.test.ts verifying that returning a plain object propagates the correct error message through default exception handling.

/claim #30