Enable API handler by default as described in issue #272

Enable API handler by default, following the discussion in issue #272 (https://github.com/cloudflare/chanfana/issues/272). The feature should ensure that ApiException (and its subclasses) are automatically handled, so developers do not need to implement a global error handler manually. Consider integrating the handler logic into OpenAPIRoute, such as a default implementation for handleApiException.

Reference for implementation and expected behavior: [issue #272](https://github.com/cloudflare/chanfana/issues/272).

Start with issue #272 to understand the expected behavior, then inspect OpenAPIRoute and its handleApiException logic. Done means ApiException and its subclasses are handled automatically by default, without requiring developers to add a global error handler.

Written by the indexing model from the issue text.