test(cli): cover cors XML alias gaps

[overtrue]

Summary

This PR adds focused regression coverage for two untested CORS paths introduced by the recent bucket CORS feature work.

The first gap was XML normalization when optional header fields are present but blank. The JSON parser path already had coverage for trimming empty allowedHeaders and exposeHeaders, but the XML path did not. This matters because rc bucket cors set accepts both JSON and XML inputs, and users can reasonably provide empty XML elements from exported or hand-edited configs.

The second gap was the deprecated top-level rc cors get alias. The newer nested rc bucket cors get path already had parser coverage, and rc cors remove was covered, but the top-level get alias was not. This PR verifies that the compatibility path still resolves to the list command and preserves the bucket argument.

Validation

I attempted to run make pre-commit as requested by the automation, but this repository currently has no Makefile and no pre-commit target. I therefore ran the documented repository checks directly:

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

All of the above passed.