Description

S3 express canary exposed a bug introduced in smithy-rs#3457 where the code overwrote the regular SigV4 session token name with the S3 Expression session token name when it shouldn't.

    4: Error { code: "InvalidRequest", message: "CreateSession request should not include \"x-amz-s3session-token\"", aws_request_id: "01c0c8864e00018e20558a130509f37740b906e4", s3_extended_request_id: "DGTJhRqVMbZdAHQ" }

For APIs like ListDirectoryBuckets or CreateSession, we should not overwrite x-amz-security-token with x-amz-s3session-token in the request header.

In the said PR, aws_sdk_s3::s3_express::utils::for_s3_express did not take into account auth schemes attached to a resolved endpoint, failing to detect that it should not override the session token name for the said APIs. This PR will resolve that issue.

Testing

• Added an integration test verifying the fix (this test currently fails when run in the destination branch, ysaito/s3express)
• Verified all canary (including wasm, s3express) passed

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