Add WpContextualExcludeFromFields attribute

[oguzkocer]

Summary

• Add WpContextualExcludeFromFields attribute to exclude fields from generated field enums
• Fix integration-tests feature warning in wp_contextual crate

Use Case

Useful for fields like additional_fields that should be included in contextual types but excluded from field filtering enums (e.g., for serde flattened catch-all fields).

Testing Instructions

1. Add a test field to comments::SparseComment:

   #[WpContext(edit, embed, view)]
   #[WpContextualExcludeFromFields]
   pub test_field: Option<String>,

2. Run: cargo expand -p wp_api comments > generated_comments.rs
3. Verify test_field appears in contextual types but NOT in SparseCommentFieldWithEditContext enum