Align importer XML namespace and validation docs

[dereuromark]

Summary

• change importer XML output to use the canonical php-collective-dto namespace
• add importer coverage for the emitted XML namespace
• improve validation docs with concrete adapter examples for framework-native validators

Verification

• composer cs-fix
• vendor/bin/phpunit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.70%. Comparing base (2ba4c82) to head (8e859c2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #105   +/-   ##
=========================================
  Coverage     82.70%   82.70%           
  Complexity     1486     1486           
=========================================
  Files            42       42           
  Lines          3660     3660           
=========================================
  Hits           3027     3027           
  Misses          633      633           

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