update: upgrade test dependencies

[inureyes]

Summary

• Upgrade mockall from 0.14.0 to 0.15.0 and serial_test from 3.5.0 to 4.0.1, including their derive crates.
• Document why crypto-common 0.1.7 intentionally keeps the transitive generic-array dependency at 0.14.7 instead of forcing the deprecation-only 0.14.9 release.
• Keep the stable Argon2 authentication stack rather than introducing a release candidate or local dependency override solely to change the transitive version.

Validation

• rustup run 1.96 cargo check --workspace --all-targets --locked
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --lib --bins --tests -- --skip test_localhost_multiple_file_upload --skip test_parallel_execution_with_multiple_nodes --skip test_download_with_unique_filenames

Risk

• Runtime dependencies and application code are unchanged.
• The three excluded localhost integration cases remain blocked by the pre-existing local ~/.ssh/known_hosts key conflict.