feat(request,response): add borrowed parts accessors

[howardjohn]

• The description, docs, and comments (words meant for humans) are written by a human, not by an LLM. (https://seanmonstar.com/micro/20260729-i-want-your-own-words/)

This adds a smaller helper to expose &Parts. Today you can only do into_parts() but requires ownership.

My main motivation is I have various helpers that require &Parts. Today this is called by something that owns req and does into_parts(); but now I have a &Request so I cannot use it. Fixing this requires duplicating the same function for Request and Parts