fix: fixes the UploadPart failure with no precalculated checksum header for FULL_OBJECT checksum type

Fixes #1342

This PR includes two main changes:

1. It fixes the case where `x-amz-checksum-x` (precalculated checksum headers) are not provided for `UploadPart`, and the checksum type for the multipart upload is `FULL_OBJECT`. In this scenario, the server no longer returns an error.

2. When no `x-amz-checksum-x` is provided for `UploadPart`, and `x-amz-sdk-checksum-algorithm` is also missing, the gateway now calculates the part checksum based on the multipart upload's checksum algorithm and stores it accordingly.

Additionally, the PR adds integration tests for:

* The two cases above
* The case where only `x-amz-sdk-checksum-algorithm` is provided

backend/posix/posix.go

@@ -2401,7 +2401,8 @@ func (p *Posix) UploadPart(ctx context.Context, input *s3.UploadPartInput) (*s3.
 
 	// If checksum isn't provided for the part,
 	// but it has been provided on mp initalization
-	if hashRdr == nil && chErr == nil && checksums.Algorithm != "" {
+	// and checksum type is 'COMPOSITE', return mismatch error
+	if hashRdr == nil && chErr == nil && checksums.Type == types.ChecksumTypeComposite {
 		return nil, s3err.GetChecksumTypeMismatchErr(checksums.Algorithm, "null")
 	}
 
@@ -2414,6 +2415,18 @@ func (p *Posix) UploadPart(ctx context.Context, input *s3.UploadPartInput) (*s3.
 		}
 	}
 
+	// if no checksum algorithm or precalculated checksum is
+	// provided, but one has been on multipart upload initialization,
+	// anyways calculate and store the uploaded part checksum
+	if hashRdr == nil && checksums.Algorithm != "" {
+		hashRdr, err = utils.NewHashReader(tr, "", utils.HashType(strings.ToLower(string(checksums.Algorithm))))
+		if err != nil {
+			return nil, fmt.Errorf("initialize hash reader: %w", err)
+		}
+
+		tr = hashRdr
+	}
+
 	_, err = io.Copy(f, tr)
 	if err != nil {
 		if errors.Is(err, syscall.EDQUOT) {