test(event): reject slash-only bucket path

[overtrue]

Summary

This change closes a small test gap in the recent bucket event command work. The event bucket path parser accepted inputs like alias///, because it trimmed trailing slashes after splitting but did not re-check whether the resulting bucket segment was still non-empty.

In practice that meant an invalid path could make it past argument parsing and fail later in a less precise code path. The cors command already rejects the same shape, so the event command was inconsistent with adjacent bucket command behavior.

Root cause

parse_bucket_path in crates/cli/src/commands/event.rs only validated the raw second path segment before trimming trailing slashes. For an input such as local///, the pre-trim segment was non-empty, but the normalized bucket name became empty.

Fix

The parser now trims trailing slashes into a local bucket variable and rejects the path when that normalized bucket value is empty. A focused unit test was added to prove that local/// is invalid.

Validation

I first added the new assertion and verified it failed against the current implementation with:

• cargo test -p rustfs-cli parse_bucket_path_error -- --nocapture

After the fix, I re-ran focused and repo-wide validation:

• cargo test -p rustfs-cli commands::event:: -- --nocapture
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

I also attempted make pre-commit per the automation constraint, but this checkout does not contain a pre-commit make target on origin/main.