feature(zebra-chain): add new Bounded vec type and apply it in AdjustDifficulty

[syszery]

Motivation

This PR fixes #8484 by introducing the new type, Bounded, which extends the existing AtLeastOne type to enforce both minimum and maximum size constraints. This ensures that collections in AdjustedDifficulty have a controlled size, improving correctness and safety compared to plain Vecs.

Solution

• Introduce pub struct Bounded<T, const MAX: usize> as a wrapper around AtLeastOne<T>.
• Bounded enforces a lower bound of at least one element (inherited from AtLeastOne) and adds an upper bound (MAX).
• Replace Vecs in AdjustedDifficulty with Bounded where constrained numbers of elements are expected.

PR Checklist

• The PR name is suitable for the release notes.
• The PR follows the contribution guidelines.
• The library crate changelogs are up to date.
• The solution is tested.
• The documentation is up to date.

[arya2]

It might be better to use bounded-vec instead of rolling our own.

[syszery]

Update

Hi @arya2, thanks for the suggestion. I’ve reworked the PR to use the bounded_vec crate.

Here’s what I did:

• Switched from the custom Bounded and AtLeastOne to the bounded-vec crate.
• Refactored AtLeastOne to be a type alias for BoundedVec.
• Updated all affected modules and tests to use BoundedVec.

The rebase is done, and I’ve confirmed that all tests, Clippy, and formatting checks passed. Let me know if you have any feedback!