Update Cloud Connected Model Metadata Resolution to Support User Model Selection

Summary

Update resolve_cloud_connected_model_metadata method in DuoAgentPlatformModelMetadataService to support user-selected model metadata, aligning it with the .com implementation.

File to Modify

ee/app/services/ai/duo_workflows/duo_agent_platform_model_metadata_service.rb

Changes Needed

Update the resolve_cloud_connected_model_metadata method to return user_selected_model_metadata, following the same pattern as resolve_gitlab_com_model_metadata.

This should support both:

1. Instance-level model selection (admin sets instance defaults)
2. User model selection (users can override instance defaults)

Current Behavior The cloud connected path doesn't respect user model selection.

Expected Behavior Cloud connected instances should support user model selection, consistent with .com behavior.