test(s3): cover Aliyun bucket lookup fallbacks

[overtrue]

Related issue(s)

None. This is automation-driven test gap coverage for the recent Aliyun OSS bucket lookup change.

Background

PR #151 changed the S3 client so bucket_lookup = auto keeps path-style addressing for most S3-compatible endpoints, but switches to virtual-hosted style for Aliyun OSS service endpoints. The merged change covered the main Aliyun and custom-endpoint paths, but a few fallback branches remained untested.

Solution

Add focused rc-s3 unit coverage for the new bucket lookup helper:

• Non-OSS aliyuncs.com services keep path-style addressing.
• Invalid endpoint strings keep path-style addressing.
• Unknown bucket_lookup values keep the conservative path-style fallback.

Test status

• cargo test -p rc-s3 bucket_lookup --lib
• make pre-commit