Fix Settings Catalog assignment-only updates

[david6983]

Summary

Fixes #266

• Apply Settings Catalog assignment updates when assignments are the only change.
• Preserve Intune-side groupId during update comparisons so group-to-allDevices/allUsers replacements are detected correctly.
• Return assignment update payloads for removal-only diffs, including removing all assignments.
• Add regression coverage for assignment removals, assignment type changes, and same-group no-op comparisons.

Testing

• .venv/bin/python -m unittest tests.test_base_graph_module
• .venv/bin/python -m unittest discover tests

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