(#2607) Wan MoE validation improvements

[bghira]

This pull request adds support for paired-stage validation for Wan 2.2 and compatible staged flavours (such as AnimeGen), allowing validation to use both stages and switch denoisers at the configured boundary. It introduces a new configuration option, updates the model and SDK to support this feature, and adds comprehensive tests and documentation in multiple languages.

Support for paired-stage validation in Wan 2.2:

• Added a new configuration option --wan_validation_load_other_stage to enable loading the opposite stage during validation, with documentation in English, Spanish, Hindi, Japanese, Portuguese, and Chinese (OPTIONS.md, OPTIONS.es.md, OPTIONS.hi.md, OPTIONS.ja.md, OPTIONS.pt-BR.md, OPTIONS.zh.md). [1] [2] [3] [4] [5] [6]
• Registered the new option in the field registry for the SDK, including UI label, help text, and documentation reference (simpletuner/simpletuner_sdk/server/services/field_registry/sections/validation.py).

Model and pipeline enhancements:

• Updated the Wan model class to support multi-stage validation, with logic to load the peer stage and switch denoisers, and added the SUPPORTS_MULTISTAGE_VALIDATION flag and related methods (simpletuner/helpers/models/wan/model.py). [1] [2] [3]
• Exposed the new configuration option in environment variable mapping (simpletuner/helpers/configuration/env_file.py).

Testing and validation:

• Added comprehensive unit tests for the new multi-stage validation logic in the Wan model, covering pipeline setup, configuration, and cache management (tests/test_wan_model.py). [1] [2] [3]
• Added a test to ensure the service reports Wan as supporting multi-stage validation (tests/test_models_service_validation_preview.py).