Adapt pyoaev to the OpenAEV asset taxonomy remodel (asset_* fields, AI target as asset) #307

Context

OpenAEV is landing an asset taxonomy remodel (OpenAEV-Platform/openaev#6704):

• The AiTarget entity is merged into Asset (asset_category = AI_TARGET, ai_target_* fields). The /api/ai_targets facade remains.
• Network-related fields move from the Endpoint level to the Asset level with a wire rename:
  • endpoint_hostname -> asset_hostname
  • endpoint_ips -> asset_ips
  • endpoint_mac_addresses -> asset_mac_addresses
  • endpoint_seen_ip -> asset_seen_ip
  • endpoint_url -> asset_url
• endpoint_platform / endpoint_arch stay endpoint-scoped (agent-capable hosts only).
• Asset groups can now contain any asset type (including AI targets), both statically and via dynamic filters on the renamed asset_* keys.

Changes needed in pyoaev

• EndpointManager create/upsert attributes must send the asset_* keys.
• AiTargetManager must treat AI targets as assets (asset_id identifier, asset_name + ai_target_* attributes).
• New ContractAiTarget contract field type (ai-target) so injector contracts can render a platform AI target picker.
• Tests updated accordingly.

This is a breaking change for any consumer building endpoint payloads with the old endpoint_* keys.