uv backend: completion fails in some cases

[AndydeCleyre]

See:

• Zsh completion: Options and flags are not completed when they come after ordered arguments astral-sh/uv#3249
• How to make shell completion work for flags/args after non-flags/args values? clap-rs/clap#4648
• Stablize Rust-Native Completion Engine Tracking Issue clap-rs/clap#3166