Update RAPx toolchain to nightly-2026-02-05

[hxuhack]

This pr mainly follows the latest verify-rust-std toolchain update.

• Bumped RAPx CI to RAPx 0.7.50 on nightly-2026-02-05.
• Slightly refined several contracts related to Challenge 17 to make them more compact and accurate.

[feliperodri]

@hxuhack @DiuDiu777 we don't have any member from RAPx as part of the committee today to help us review any related updates to RAPx in the contest. Who should we nominate in the RAPx side? It'd be nice if the person also could be tagged to review some challenges as well (of course respecting your time restrictions).

[DiuDiu777]

Hi, @feliperodri. Thanks for bringing this up.

I’d be happy to serve as the RAPx representative on the committee and help review RAPx-related updates and relevant challenges.

[feliperodri]

@hxuhack could you make a change to our https://github.com/model-checking/verify-rust-std/blob/main/.github/pull_requests.toml doc and include @DiuDiu777 in it? This will officially add them to the committee.

[hxuhack]

@feliperodri I’ve updated the PR and added @DiuDiu777 to the committee. Thanks!