test(s3): cover batch delete partial response

[overtrue]

Summary

This adds focused regression coverage for the batch delete path introduced with the recent RustFS force-delete work.

The new delete_objects_with_options helper now handles RustFS-specific request options and parses S3 batch delete responses. The existing tests cover the force-delete header behavior, but they do not exercise the branch where the backend returns a mixed response with both <Deleted> and <Error> entries. That left a gap in the changed code where a future refactor could incorrectly fail the whole request or drop successfully deleted keys.

Root cause

The follow-up test coverage around rm --purge concentrated on request construction and the full purge flow. It did not assert the non-fatal partial-success behavior already implemented in delete_objects_with_options.

Fix

This PR adds one narrow unit test in crates/s3/src/client.rs that feeds a mixed DeleteResult payload into the captured request client and verifies two things:

1. the batch delete request is still issued to the expected delete endpoint;
2. the client returns only the successfully deleted keys instead of treating the partial response as a hard failure.

The change is test-only and stays inside the recently modified S3 delete helper.

Validation

I ran the required checks successfully:

• cargo test -p rc-s3 delete_objects_with_partial_errors_returns_deleted_keys --lib
• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

I also ran make pre-commit per the automation instruction, but this checkout does not define that target and make returned No rule to make target 'pre-commit'.