fix: Update OpenFeature Provider user ID mapping priority and add userId support

[jonathannorris]

Summary

Enhances the OpenFeature Provider to support userId field mapping and corrects the priority order to match other DevCycle SDKs.

Changes

• Priority Order: Updated to targetingKey → user_id → userId (was previously backwards)
• New Field Support: Added support for userId field mapping from OpenFeature evaluation context
• Validation: Added robust validation for user ID fields (required, must be string, non-empty)
• Error Handling: Improved error messages to include specific field names for better debugging
• Custom Data: All user ID fields (targetingKey, user_id, userId) are now excluded from custom data

Validation Added

• Throws ArgumentError if no user ID field is provided
• Throws ArgumentError if user ID field is not a string (with field name)
• Throws ArgumentError if user ID field is empty

Tests

• ✅ 45 comprehensive tests covering all scenarios
• ✅ Priority order validation
• ✅ Field validation edge cases
• ✅ Custom data exclusion behavior
• ✅ Backward compatibility

This change aligns the Ruby SDK with other DevCycle OpenFeature providers and ensures consistent behavior across all platforms.

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