fix: Support nested enveloped signature location (#525)

Update XPath query to find Signature elements at any depth within the
document, not just direct children. This fixes an issue where signatures
nested within other elements were not properly detected and removed.

Author: shunkica

src/enveloped-signature.ts

@@ -17,7 +17,7 @@ export class EnvelopedSignature implements TransformAlgorithm {
   process(node: Node, options: TransformAlgorithmOptions): Node {
     if (null == options.signatureNode) {
       const signature = xpath.select1(
-        `./*[local-name(.)='Signature' and namespace-uri(.)='${XMLDSIG_URIS.NAMESPACES.ds}']`,
+        `.//*[local-name(.)='Signature' and namespace-uri(.)='${XMLDSIG_URIS.NAMESPACES.ds}']`,
         node,
       );
       if (isDomNode.isNodeLike(signature) && signature.parentNode) {