Updated custom roles payload for bypass options

What does this MR do and why?

This change modifies how custom roles are stored and handled in a policy exceptions feature. Previously, custom roles were stored as simple ID numbers in an array. Now they're stored as objects containing an ID property (like {id: 1} instead of just 1).

The code was updated in two main areas: when displaying existing custom roles, it now extracts just the ID numbers from these objects for display purposes, and when saving new custom role selections, it wraps the ID numbers in objects before storing them. The tests were also updated to reflect this new data structure, ensuring the feature continues to work correctly with the new format.

References

Screenshots or screen recordings

Description	UI
Custom roles	custom.mov

How to set up and validate locally

Enable feature flag:

Feature.enable(:security_policies_bypass_options)
Feature.enable(:security_policies_bypass_options_group_roles)

• Secure -> Policies
• New merge request approval policy
• Open Advanced settings
• Click add exception button
• Select roles option (create custom roles if there are no custom roles)
• Check when custom role is selected it is selected as { id: int } in yaml

MR acceptance checklist

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Related to #560031 (closed)