test(s3): cover batch delete default force header

[overtrue]

Summary

This change adds focused coverage for the batch delete path introduced by the RustFS force-delete support.
The recent rm --purge work already verified that batched deletes send the custom header when force delete is enabled, but it did not cover the opposite branch.
Without this test, a regression could silently start sending the RustFS-specific header on ordinary batch deletes.

Root cause

The feature added conditional header wiring in both single-object and batch delete requests.
Only the positive batch case was exercised, so the default path remained unverified.

Fix

Add a unit test in rc-s3 that issues delete_objects_with_options with DeleteRequestOptions::default() and asserts that x-rustfs-force-delete is omitted.
The change stays within the recent delete-path work and does not alter runtime behavior.

Validation

• cargo test -p rc-s3 delete_objects_without_force_delete_omits_rustfs_header
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Notes

make pre-commit is not available in this repository, so validation used the equivalent CI commands defined in AGENTS.md and .github/workflows/ci.yml.