feat: @W-22143475 - Add AiTestingDefinition to metadata registry (#1748)

Add support for AiTestingDefinition metadata type with deploy/retrieve
capabilities (source tracking not supported).

Changes:
- Added AiTestingDefinition type definition to metadataRegistry.json
- Added aiTestingDefinition suffix mapping
- Updated METADATA_SUPPORT.md to show as supported with tracking limitation

Author: WillieRuemmele