fix(xml): preserve UTF-8 declaration in signed DPS output

[vitormattos]

Summary

Preserves an explicit UTF-8 XML declaration in signed DPS payloads returned by DpsSigner.

Why

Some downstream integrations expect the signed XML to retain a deterministic declaration line (<?xml version="1.0" encoding="UTF-8"?>).

Changes

• src/Xml/DpsSigner.php
  • serialize only the document element
  • prepend explicit UTF-8 declaration
  • keep fallback to original input when serialization fails
• tests/Unit/Xml/DpsSignerTest.php
  • add regression test asserting UTF-8 declaration prefix

Validation

• vendor/bin/phpunit --filter='DpsSignerTest' --no-coverage
  • Passed: 10 tests, 63 assertions