What bug does this fix? Explain your changes.

The Bug

On WordPress VIP and similar platforms where intermediate image sizes are disabled, MediaItemSizeEnum was being registered with zero values. This creates an invalid GraphQL schema because the GraphQL specification requires that enums must define at least one value.

Error Message:

Enum type MediaItemSizeEnum must define one or more values.

This breaks:

• Schema introspection
• Code generation tools (e.g., graphql-codegen)
• Any GraphQL client that validates schemas

Root Cause

The breaking change introduced in commit e5a9fe0 removed fallback support for image sizes. The get_image_sizes() method now solely relies on get_intermediate_image_sizes(), which returns an empty array on WordPress VIP (and possibly other environments) where intermediate sizes are disabled by platform policy.

The Fix

Added fallback logic to ensure MediaItemSizeEnum always has at least one value:

1. When get_intermediate_image_sizes() returns empty, populate with standard WordPress image sizes: thumbnail, medium, medium_large, large, and full
2. Special handling for full size (0x0 dimensions since it represents the original uploaded image)
3. These enum values act as "hints" that WordPress core functions (like wp_get_attachment_image_src()) gracefully resolve at runtime

This maintains backward compatibility while ensuring schema validity on all platforms.

Does this close any currently open issues?

Closes #3432

Testing Strategy

This PR follows the recommended commit structure:

Commit 1: Failing Test

Commit: 58fc33a - Add test reproducing MediaItemSizeEnum empty values bug on VIP

The test simulates VIP environment by filtering get_intermediate_image_sizes to return empty array, then performs schema introspection to verify MediaItemSizeEnum has at least one value.

This commit will show failing tests in CI ❌

Commit 2: Implementation Fix

Commit: 7db8c1bd - Add fallback image sizes to MediaItemSizeEnum

Implements the fix by adding fallback logic to get_image_sizes() method with comprehensive inline documentation.

This commit will show passing tests in CI ✅

Test Results

• Failing Test: Commit 58fc33a demonstrates the bug with a failing test (see: https://github.com/wp-graphql/wp-graphql/actions/runs/18928825922)
• Passing Tests: Commit 7db8c1bd resolves the bug and now the tests all pass (see: https://github.com/wp-graphql/wp-graphql/actions/runs/18928894854)

Before/After Examples

Before (Buggy Behavior):

On WordPress VIP with no intermediate sizes:

query IntrospectMediaItemSizeEnum {
  __type(name: "MediaItemSizeEnum") {
    name
    enumValues {
      name
    }
  }
}

Result: Schema validation error - Enum type MediaItemSizeEnum must define one or more values

After (Fixed Behavior):

query IntrospectMediaItemSizeEnum {
  __type(name: "MediaItemSizeEnum") {
    name
    enumValues {
      name
      description
    }
  }
}

Result:

{
  "data": {
    "__type": {
      "name": "MediaItemSizeEnum",
      "enumValues": [
        { "name": "THUMBNAIL", "description": "Small image preview suitable for thumbnails and listings. (150x150)" },
        { "name": "MEDIUM", "description": "Medium image preview typically suitable for listings and detail views. (300x300)" },
        { "name": "MEDIUM_LARGE", "description": "Medium-to-large image preview suitable for listings and detail views. (768x0)" },
        { "name": "LARGE", "description": "Large image preview suitable for detail views. (1024x1024)" },
        { "name": "FULL", "description": "Full-size image." }
      ]
    }
  }
}

Additional Context

• No Breaking Changes: This fix is fully backward compatible
• Performance: No performance impact - fallback only activates when get_intermediate_image_sizes() returns empty
• Platform Support: Specifically fixes WordPress VIP but benefits any platform that disables intermediate sizes
• WordPress Core Compatibility: The enum values serve as hints; WordPress core functions already handle missing sizes gracefully by falling back to available sizes

Documentation Added

Comprehensive inline comments explain:

• Why fallbacks are necessary (GraphQL spec, VIP platform)
• How WordPress resolvers handle the enum values
• Reference to issue MediaItemSizeEnum becomes empty when intermediate image sizes are disabled → schema invalid #3432