defense-in-depth check for <Error> in CompleteMultipartUpload respons…

[cloud4t0r]

Issue #, if available:

The issue stems from a documented edge case by AWS: when you perform a CompleteMultipartUpload, S3 may return an HTTP 200 OK with an XML body containing an instead of the expected . This typically occurs during internal S3 overloads (InternalError). Without this patch, the SDK treated the 200 as a success and continued, even though the multipart upload had actually failed.

this patch address this todo in the original code :
/**
* TODO: The body of the response can be ERROR, check Error specified in body part from
* https://docs.aws.amazon.com/AmazonS3/latest/API/API_CompleteMultipartUpload.html#AmazonS3-CompleteMultipartUpload-response-CompleteMultipartUploadOutput
* We need to handle this case.
* TODO: the checksum returned within the response of complete multipart upload need to be exposed?
*/

Description of changes:
The patch adds three protections in the AWS_S3_AUTO_RANGED_PUT_REQUEST_TAG_COMPLETE_MULTIPART_UPLOAD handler:

Detection of <Error><Code> in the body: Even if error_code == AWS_ERROR_SUCCESS (HTTP 200), we parse the XML body to look for an <Error><Code>. If found, we map the S3 error to a CRT error code (or AWS_ERROR_S3_NON_RECOVERABLE_ASYNC_ERROR if unknown). This is an extra safety net in addition to the generic mechanism in s3_meta_request.c, which already performs this check beforehand but might miss it in some timing cases (body arriving after send_request_finish).

Verification of the ETag presence: On a real success, the body must contain a <CompleteMultipartUploadResult> with an <ETag>. If we get a 200 but no ETag (empty body, truncated body, or unexpected content), it must be an issue → we force AWS_ERROR_S3_NON_RECOVERABLE_ASYNC_ERROR.

Conditional invocation of the headers_callback: Before the patch, the user callback (headers_callback) was always invoked on a 200, even if the body contained an error. Now it is only called if error_code is still AWS_ERROR_SUCCESS after the two checks above. This prevents notifying the caller of a "success" that is not actually one.

In summary: this is defense‑in‑depth so that the calling code never treats a multipart upload as successful if S3 has returned an error disguised as a 200 OK.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.