chunked_vector: Various improvements

[BenPope]

Some improvements to chunked_vector:

• Satisfy random_access_iterator concept

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.2.x
• v25.1.x
• v24.3.x

Release Notes

• none

[Copilot]

Pull Request Overview

This PR improves the chunked_vector iterator implementation to properly satisfy the C++20 random access iterator concept requirements. The changes ensure the iterator type aliases are correctly defined to support both const and non-const iterators while handling the special case of bool type.

Key Changes

• Added iterator_concept type alias to explicitly declare random access iterator support
• Fixed value_type to always be T (not conditionally const)
• Corrected pointer and reference type aliases to properly handle const/non-const variants

[rockwotj]

Can we static assert this in tests?

[BenPope]

Can we static assert this in tests?

Yeah, I think there's some more to come, but I'm not full time on it.

[dotnwat]

i don't think this will work, right? doesn't random access iterators imply certain properties about the memory layout? stuff like for iterators a and b you can do pointer arithemetic with them that acts like the memory is contiguous?

[rockwotj]

i don't think this will work, right? doesn't random access iterators imply certain properties about the memory layout? stuff like for iterators a and b you can do pointer arithemetic with them that acts like the memory is contiguous?

There should be https://en.cppreference.com/w/cpp/iterator/contiguous_iterator.html for the difference between random access and contiguous, but maybe not everything respects that properly

[dotnwat]

i don't think this will work, right? doesn't random access iterators imply certain properties about the memory layout? stuff like for iterators a and b you can do pointer arithemetic with them that acts like the memory is contiguous?

There should be https://en.cppreference.com/w/cpp/iterator/contiguous_iterator.html for the difference between random access and contiguous, but maybe not everything respects that properly

Not sure, but I the second bullet point under semantic requirements here https://en.cppreference.com/w/cpp/iterator/random_access_iterator.html is where I saw the pointer difference idea.