fix(admin): start full heal when bucket omitted

[cxymds]

Summary

• Make rc admin heal start <alias> send a recursive root heal request when no bucket is provided.
• Keep bucket and prefix heal requests non-recursive, and keep heal stop using the existing non-recursive root request body.
• Update the heal start help text and add coverage for the no-bucket recursive request.

Verification

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