[Backport version-15] feat: implement XSD-driven OpenImmo export & validation

[github-actions]

Backport of #16 to version-15.

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Description:

• Feature: Replaced template-based XML generation with a robust XSD-driven builder for OpenImmo.
• Fixes:
  • Resolved schema validation errors (missing attributes in anhang, invalid aktionart values, and empty mandatory tags).
  • Added dynamic image handling for child tables (custom_image_gallery).
  • Implemented strict data validation to ensure XML compliance.
• Configuration: Added openimmo_anid field to Integration Source and updated dependencies (lxml==6.1.0).
• Version: Bumped app version to 0.0.2.