fix(s3): preserve bucket service error details

[overtrue]

Related issue

Fixes #210.

Evidence / reproduction

Issue #210 shows both rc mb local/test/ and rc ls local returning only Network error: service error against a RustFS endpoint. The bucket-level SDK paths were using SdkError::to_string(), which can collapse service responses to that generic text.

Root cause

list_buckets and create_bucket did not use the existing SDK error formatter, so backend error codes from headers such as x-amz-error-code were lost.

Fix

Use the shared format_sdk_error helper for bucket listing and creation errors, and add regression tests proving service error codes are preserved.

Validation

• cargo test -p rc-s3 preserves_service_error_code --lib
• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace
• git diff --check

[Copilot]

Pull request overview

This PR fixes loss of backend S3 service error details for bucket-level operations by routing list_buckets and create_bucket failures through the shared SDK error formatter, ensuring service error codes (e.g., from x-amz-error-code) are preserved in user-facing errors.

Changes:

• Use S3Client::format_sdk_error when mapping AWS SDK errors for list_buckets and create_bucket.
• Add regression tests asserting that bucket listing/creation errors include the service error code in the resulting Error::Network message.

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