feat: add XML serialization support for Pydantic models

[mischadiehm]

Summary

This PR adds XML serialization capabilities to XMLPydantifyModel, enabling Pydantic models to be serialized to XML with proper namespace handling.

Changes

New Features

• XMLPydantifyModel.model_dump_xml(): Recursive XML generation with:

  • Proper namespace declarations (only when namespace differs from parent)
  • Wrapper model passthrough for single-field models
  • List field expansion (each item becomes separate element)
  • Optional field handling (None values skipped)

• model_dump_xml_string(): String serialization helper with:

  • Pretty-print support
  • Optional NETCONF <data> root wrapper

Exports

• XMLPydantifyModel
• model_dump_xml_string
• NETCONF_BASE_NS

Testing

• 16 new unit tests covering all features
• 2 integration tests validating against expected XML output
• All tests passing (18/18)
• Linter passing

Files Changed

• src/pydantify_common/model.py - Core serialization logic
• src/pydantify_common/helper.py - String helper function
• src/pydantify_common/__init__.py - Updated exports
• tests/test_model_dump_xml.py - New unit tests
• tests/test_examples.py - Updated integration tests

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