test(cli): cover cors help discoverability

[overtrue]

Summary

The recent bucket CORS feature added two new discoverability surfaces: the deprecated top-level rc cors command and the nested rc bucket cors command group. Both command paths were covered by parser tests, but the CLI help contract did not assert that they remained visible in --help output.

This PR adds focused help-contract coverage in crates/cli/tests/help_contract.rs for those recent command surfaces. The top-level command help now must continue to list cors and show its deprecation guidance, and the nested bucket cors help now must continue to advertise the expected list, set, and remove subcommands.

Root Cause

The original feature implementation verified command parsing and some option-level behavior, but it did not add help-contract assertions for the new command entry points. That left a small regression window where future Clap wiring changes could make the commands harder to discover without failing tests.

Validation

I attempted to run make pre-commit, but this checkout does not define that target:

• make pre-commit -> make: *** No rule to make target \pre-commit'. Stop.`

I then ran the repository's required checks directly:

• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

I also ran the focused help-contract checks while iterating on the assertions:

• cargo test --test help_contract top_level_command_help_contract -- --exact
• cargo test --test help_contract nested_subcommand_help_contract -- --exact