fix replication of checksum when encryption is enabled (#20161)

- Adding functional tests
- Return checksum header on GET/HEAD, previously this was returning
  InvalidPartNumber error

cmd/object-handlers-common.go

@@ -248,10 +248,19 @@ func checkPreconditions(ctx context.Context, w http.ResponseWriter, r *http.Requ
 	}
 
 	// Check if the part number is correct.
-	if opts.PartNumber > 1 && opts.PartNumber > len(objInfo.Parts) {
-		// According to S3 we don't need to set any object information here.
-		writeErrorResponse(ctx, w, errorCodes.ToAPIErr(ErrInvalidPartNumber), r.URL)
-		return true
+	if opts.PartNumber > 1 {
+		partFound := false
+		for _, pi := range objInfo.Parts {
+			if pi.Number == opts.PartNumber {
+				partFound = true
+				break
+			}
+		}
+		if !partFound {
+			// According to S3 we don't need to set any object information here.
+			writeErrorResponse(ctx, w, errorCodes.ToAPIErr(ErrInvalidPartNumber), r.URL)
+			return true
+		}
 	}
 
 	// If-None-Match : Return the object only if its entity tag (ETag) is different from the