Add variants to include parameter documentation for Products endpoints

[Copilot]

Problem

The ?include parameter for the Get All Products endpoint supports a variants value that was not documented in the OpenAPI schema. This creates inconsistency between the actual API behavior and the documentation.

Solution

Added variants to the IncludeParamBase schema enum in reference/catalog/products_catalog.v3.yml. This parameter schema is used by three endpoints:

• GET /catalog/products (Get All Products)
• GET /catalog/products/{product_id} (Get a Product)
• PUT /catalog/products/{product_id} (Update a Product)

Context

The variants value was already:

• Supported by the API implementation
• Included in the response schemas (productVariant_Full)
• Documented in a different parameter (include_fields)

This change brings the include parameter documentation in line with the actual API behavior and makes it consistent with the existing include_fields parameter.

Testing

• ✅ OpenAPI spec validation passes (Spectral)
• ✅ MDX linting passes
• ✅ No breaking changes to existing functionality

Fixes the documentation gap identified in the original issue where variants was a supported but undocumented value for the include parameter.

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