feat: Handle preprocess errors

[josecriane]

Summary

Reworks erf_router:handle/2 so that requests failing at the preprocess stage still flow through the postprocess middlewares and postprocess/1, instead of returning the raw 400 response early.

Motivation

Previously, when preprocess/1 failed (e.g. an unreadable request), handle/2 returned the 400 Bad Request response directly, bypassing apply_postprocess_middlewares/3 and postprocess/1. This caused two problems:

• Postprocess middlewares (logging, metrics, header injection, etc.) never ran for malformed requests, leaving a gap in observability and inconsistent response handling.
• The error response was returned as-is, with no chance to be handled by the postprocessing pipeline, so it could end up not matching the error shape defined in the OpenAPI spec.

Changes

• Moved the preprocess/1 case so it only computes the {InitialResponse, InitialRequest} pair.
• On preprocess error, the 400 response is paired with the original RawRequest ({ResponseError, RawRequest}) so downstream stages have a valid request to work with.
• apply_postprocess_middlewares/3 and postprocess/1 now run unconditionally for both the success and error paths.

[javiergarea]

LGTM! 🚀