[messaging] feat: fix multiple error message handler

[Sebastian-Iwanczyszyn]

Motivation

Fixes error handling in HandlerMiddleware when multiple handlers are registered for the same routing key and one of them fails asynchronously.

Previously, the single-handler path wrapped errors correctly, but the multi-handler path assumed every rejected result from Promise.allSettled() had the shape { handler, error }. That was only true for synchronous throws caught inside the
mapper. For normal async rejections, result.reason was the raw Error, which caused follow-up handling to read undefined and break error logging/aggregation.

What Changed

• Normalized multi-handler execution so both sync throws and async rejections are wrapped consistently with handler metadata
• Preserved existing behavior for successful handlers and aggregated HandlersException
• Added a unit test covering the case where multiple handlers are registered and one rejects

Type of change

• Bug fix
• Feature
• Refactor
• Documentation
• Code improvement / refactoring
• Other (please describe)

Breaking change

• Yes
• No

Checklist

• Unit tests added/updated
• E2E tests added/updated (if applicable)
• Documentation updated if needed