chore: fix clippy lints surfaced by Rust 1.97 toolchain bump

[StefanSteiner]

What

Fix three clippy lints that were tightened/added in the Rust 1.97 stable release (2026-07-07) and now fail CI under -D warnings. All three are on pre-existing code — no behavior change.

Lint	Location	Fix
dead_code	hyperdb-api/tests/common/mod.rs	Replace the scattered per-item #[expect(dead_code)] / #[allow(dead_code)] attributes with a single module-level #![allow(dead_code, reason = ...)]. The shared helpers are compiled into every integration-test binary but each binary uses only a subset, so any given helper is dead in the binaries that don't reference it. allow (not expect) because the same item is live in other binaries — an expect would be unfulfilled there and trip unfulfilled_lint_expectations. The struct's previous #[expect] was in fact already unfulfilled under 1.97.
useless_borrows_in_formatting	hyperdb-api/tests/logging_tests.rs (×19)	&*logs → *logs in assert! format args.
manual_assert_eq	hyperdb-api/src/server_version.rs:265	assert!(v1 == ...) → assert_eq!(v1, ...).

Why now

The repo's rust-toolchain.toml pins channel = "stable" — a moving target — and CI's actions-rust-lang/setup-rust-toolchain@v1 { toolchain: stable } fetches the newest stable. main's last CI run predates the 1.95 → 1.97 bump, so main currently looks green but fails the moment CI re-runs on 1.97. Any open PR that triggers a fresh CI run (e.g. the open dependabot PRs) will hit this same wall until this lands.

Verification

On the CI toolchain (1.97.0) with HYPERD_PATH set, both CI commands are green:

cargo +1.97.0 fmt --all --check                                          # exit 0
cargo +1.97.0 clippy --workspace --all-targets --all-features -- -D warnings   # exit 0

Also verified green on 1.95 (backward-compatible), and the affected tests pass: server_version 14/0, logging_tests 8/0.