test(s3): cover bucket cors set request

[overtrue]

Related issue

No specific issue. This follows the test-gap automation for recent bucket CORS changes.

Background

Recent work added bucket CORS management through the S3 adapter. Existing unit coverage checked the core rule conversion and XML parsing paths, but did not assert that set_bucket_cors actually sends the expected PUT request and serialized CORS rule fields to the backend.

Root cause

The request-building path was only covered indirectly. A regression in the SDK request wiring, subresource URI, or serialized CORS fields could pass the existing conversion tests while still producing an incorrect backend request.

Solution

Added a focused S3 client unit test that captures the set_bucket_cors request and verifies the PUT method, CORS subresource URI, and serialized rule fields including ID, origins, methods, allowed headers, exposed headers, and max age seconds.

Validation

cargo test -p rc-s3 set_bucket_cors_sends_rule_fields

cargo fmt --all --check

cargo fmt --all && cargo clippy --workspace -- -D warnings

cargo test --workspace

make pre-commit was requested but this repository does not contain a Makefile or a pre-commit target, so the equivalent required Rust checks above were run directly.