Fix Clippy and minimal-versions CI failures on Rust 1.91

[inureyes]

Description

Clippy and Minimal-versions have been failing on main since the 1.91 toolchain reached the runners, independently of any pull request: https://github.com/Eugeny/russh/actions/runs/33791529793 at 8a51a0d.

Clippy 1.91 extended collapsible_if to nested if let, which flags 40 sites. cargo clippy --fix rewrites them as let-chains, but neither stable rustfmt nor nightly 1.10.0 reformats the result, so every body keeps its pre-collapse indentation until it is fixed by hand. That churn lands in the session and encrypted modules that most open PRs touch, so this allows the lint next to the two allows already in lib.rs rather than reformatting them now.

Under minimal versions, yasna's num-bigint = "0.4" resolves to 0.4.0, whose bits().div_ceil(&u64::from(bits)) now picks the inherent u64::div_ceil over Integer::div_ceil and fails to compile. 0.4.6 switched those three call sites to UFCS, so the floor is raised with a renamed optional dependency tied to the same feature as yasna, the same way cipher, ghash, polyval and universal-hash are already pinned in this file.

Verified with the CI commands: cargo clippy with and without --all-features, cargo minimal-versions check --all-features --no-dev-deps, cargo fmt --check, cargo test --all-features, plus the 1.89.0 wasm32-wasip1 build and an MSRV check.

AI Usage

Choose the level of AI involvement for this PR.

• Fully vibe coded
• AI-designed, AI-coded, manually checked
• Human-designed, AI-coded
• Human-designed, human-coded (includes AI autocompletions and boilerplate gen)

_{This is not to block AI contributions but rather to speed up PR review (saves time on trying to deduce the logic behind AI hallucinations).}