test(s3): cover batch delete force-delete header omission

[overtrue]

Summary

A recent force-delete change added RustFS-specific header handling for both single-object and batch-object deletes. The batch path already had a positive test for force_delete = true, but it did not verify the default path where the header must be omitted. That left room for a regression where ordinary batch deletes could start sending the RustFS purge header unintentionally.

This change adds a focused unit test in crates/s3/src/client.rs that exercises delete_objects_with_options with DeleteRequestOptions::default() and asserts that x-rustfs-force-delete is absent from the outgoing request. The scope stays limited to the recently changed codepath and does not alter production behavior.

Root Cause

The recent test coverage around force-delete support validated the enabled branch for batch deletes and both enabled and disabled branches for single-object deletes. The disabled branch for batch deletes was left uncovered.

Validation

The repository does not currently define a make pre-commit target, so I ran the CI-equivalent checks directly:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

I also ran the focused regression check:

• cargo test -p rc-s3 delete_objects_ -- --nocapture