test(s3): cover custom XML request headers

[overtrue]

Related issue(s)

None.

Background

Recent custom request header support covered AWS SDK requests and presigned URL behavior, but the manually signed XML request path used by bucket replication and related operations did not have direct regression coverage.

Solution

Add a focused S3 client unit test with a local HTTP capture server. The test verifies that custom x-amz headers are sent on XML requests and included in the Authorization signed header list.

Test status

• cargo test -p rc-s3 custom_headers_are_added_to_xml_requests_before_signing --lib
• cargo fmt --all --check
• git diff --check
• make pre-commit

[Copilot]

Pull request overview

Adds regression coverage in the S3 client unit tests to ensure user-provided custom x-amz-* request headers are applied to the manually-signed XML request path (used by replication/bucket ops) and included in the SigV4 signed header list.

Changes:

• Added a small local TCP capture server to record raw XML HTTP requests (method/target/headers).
• Added a unit test asserting custom headers are present on XML requests and appear in the Authorization signed headers list.

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