Add tests for AssignRole

[eleftherias]

Summary

This adds unit tests for AssignRole and moves the invite and role creation logic to their respective services.

Fixes #3673

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 54.314% (+0.4%) from 53.887%
when pulling 6c209c1 on eleftherias:test-assign-role
into bd4ba06 on stacklok:main.