Draft: Add model layer for AI Catalog object storage

What does this MR do and why?

Part 3 of the AI Catalog object storage MR split. !227895

Adds the model layer changes to support storing YAML definitions in object storage.

Key changes:

• Mount YamlDefinitionUploader on ItemVersion
• Add transparent yaml_definition accessor that reads from object storage when yaml_definition_file is present
• Add cleanup_version_files callback on Item destroy
• Add yaml_definition_file_store attribute and before_save callback
• Make definition validation conditional on feature flag
• Add specs for yaml_definition fallback and file cleanup

MR Breakdown Plan

MR	Description	Dependencies
1a	Add yaml_definition_file columns to ai_catalog_... (!228120 - merged)	None
1b	Add ai_catalog object storage configuration and... (!228122 - merged)	None
2	Add YamlDefinitionUploader for AI Catalog YAML ... (!228839 - merged)	1b
3 (This MR)	Model layer: Mount uploader, add transparent accessor	1a, 2
4	Service layer & schema: Upload in services, make yaml_definition optional	3

Issue: #591638