fix(s3): preserve delete bucket conflict codes

[overtrue]

Summary

Fixes #160.

This change preserves the S3 service error code when DeleteBucket fails, so rc rb can keep reporting bucket-specific failures instead of collapsing them into a generic network error.

Root cause

crates/s3/src/client.rs was converting DeleteBucket failures with e.to_string(), which drops RustFS/AWS-compatible error codes such as BucketNotEmpty from the CLI-facing path.

Solution

• format DeleteBucket failures with the existing SDK-aware formatter
• map BucketNotEmpty to Error::Conflict
• keep NoSuchBucket mapped to Error::NotFound
• add regression tests for both mappings

Verification

• make pre-commit
• manual check against rustfs/rustfs:latest
• reproduced rc rb local/<bucket> on a non-empty bucket and confirmed exit code 6 with Bucket '<alias>/<bucket>' is not empty. Use --force to delete all objects first.

Risk

N/A