test(s3): cover delete bucket network fallback

[overtrue]

Related

This pull request adds focused regression coverage for the recent delete_bucket error mapping work in #163.

Background

delete_bucket() now distinguishes NoSuchBucket and BucketNotEmpty from other backend failures. That behavior was covered for the two special-case branches, but the generic fallback path was still untested.

Root Cause

The recent change introduced a third branch that maps all remaining SDK failures to Error::Network. Without a direct test, later edits could accidentally change that fallback classification or stop surfacing the backend error code.

Solution

Add one narrow unit test beside the existing delete_bucket regression tests in crates/s3/src/client.rs. The new test drives a synthetic InternalError response and asserts that the client keeps classifying it as Error::Network while preserving the backend code in the message.

Validation

I ran the focused seam first with cargo test -p rc-s3 delete_bucket_maps_ --lib, then the full repository gate with make pre-commit.