MPT-16437 Reorganise resource catalog mixins structure

[albertsola]

Closes MPT-16437

• Reorganise catalog mixins into separate modules for clearer structure and maintainability.
• Add ActivatableMixin and AsyncActivatableMixin (mpt_api_client.resources.catalog.mixins.activatable_mixin) with activate() and deactivate() helpers.
• Add DocumentMixin and AsyncDocumentMixin (mpt_api_client.resources.catalog.mixins.document_mixin) composing create-file, download-file and publishable capabilities.
• Add MediaMixin and AsyncMediaMixin (mpt_api_client.resources.catalog.mixins.media_mixin) composing create-file, download-file and publishable capabilities.
• Refactor publishable_mixin.py to retain only PublishableMixin/AsyncPublishableMixin (removed composite mixin definitions).
• Centralise public exports in mpt_api_client.resources.catalog.mixins.init.py to re-export all mixins.
• Add focused unit tests for Activatable, Document and Publishable mixins (sync + async) in dedicated test modules.
• Remove legacy tests/unit/resources/catalog/test_mixins.py (tests migrated to module-specific files).
• Remove several per-file lint suppressions from pyproject.toml related to the reorganised files.