test(cli): cover rm purge parser

[overtrue]

Summary

This adds focused parser coverage for the --purge flag introduced on rm.

The recent rm --purge work already had behavioral coverage for the delete path and a help-contract check for flag discoverability, but the CLI parser itself still had no direct assertion that the flag is accepted through both command entrypoints that share RmArgs: the legacy rc rm form and the noun-first rc object remove form.

This change adds two parser tests in crates/cli/src/commands/mod.rs that verify both entrypoints preserve the target path and set purge = true. The scope stays limited to the recent rm change surface and does not touch production code.

Root Cause

PR #82 wired --purge into RmArgs, but parser coverage in commands::tests did not exercise that new flag. That left a small regression window where clap wiring could drift for one entrypoint without a focused unit test catching it.

Validation

make pre-commit is not defined in this checkout, so I ran the repository's required checks directly:

• cargo test -p rustfs-cli purge_flag --lib
• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace