fix(cargo): rewrite cargo nextest commands

[ychampion]

Summary

• Register cargo nextest in the rewrite registry so hooks can map it to rtk cargo nextest.
• Cover the rewrite and keep Cargo subcommand registry coverage current.

Fixes #2046

Test plan

• CARGO_INCREMENTAL=0 cargo fmt --all -- --check
• CARGO_INCREMENTAL=0 cargo clippy --all-targets
• CARGO_INCREMENTAL=0 cargo test
• Manual testing: CARGO_INCREMENTAL=0 cargo run --quiet -- rewrite "cargo nextest run" prints rtk cargo nextest run

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