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

[seanmonstar]

This has come up a few times, let me find the references... But in short, the Parts type works for into because however we store the fields in memory normally, we can always convert them into Parts. But exposing a reference would lock the layout in place.

#712
#372
#56