[bug] Resending an unchanged is_admin value deletes the OrganizationUser membership

Describe the bug

SuperUserDetailSerializer.update() in openwisp_users/api/serializers.py uses a non-obvious rule for organization_users: if the payload includes an existing membership and the submitted is_admin value is the same as the current value, the membership is deleted instead of being left unchanged.

This behavior is implemented here:

if org_user.is_admin != org_user_data["is_admin"]:
    org_user.is_admin = org_user_data["is_admin"]
    org_user.full_clean()
    org_user.save()
else:
    org_user.delete()

As a result, API clients that resend the current representation of a user on PATCH can unintentionally delete organization memberships. The only way to preserve an existing membership is to omit it entirely from the payload, or omit is_admin from that specific entry.

Why this is a problem

• The delete-vs-update behavior is not documented in the REST API docs.
• The serializer response representation includes organization_users, so a naive PATCH round-trip can be destructive.
• This is easy to miss for API consumers who expect unchanged fields to remain unchanged.

Relevant code

• openwisp_users/api/serializers.py
  • BaseSuperUserSerializer.to_representation() serializes organization_users with organization and is_admin.
  • SuperUserDetailSerializer.update() deletes the membership when is_admin matches the current value.
• docs/user/rest-api.rst currently notes that is_admin represents the organization manager flag, but does not document the delete-on-unchanged behavior.
• Existing tests such as test_remove_org_user_api and test_make_user_org_admin_api cover parts of this flow, but do not make the contract explicit in the docs.

Proposed resolution

Please make a product decision on one of these options:

1. Document the current behavior explicitly, or
2. Replace it with an explicit removal signal instead of overloading "unchanged" to mean "delete".

Option 2 would be a breaking API change for existing consumers, so it likely needs a migration plan or versioning strategy.

Notes

This is unrelated to the disabled-organization write protection work. The behavior predates that effort and appears to come from the original REST API implementation.

Start with SuperUserDetailSerializer.update() and BaseSuperUserSerializer.to_representation() in openwisp_users/api/serializers.py, then read docs/user/rest-api.rst and the tests test_remove_org_user_api and test_make_user_org_admin_api. The issue requires a product decision between documenting the current behavior and changing the removal signal; done means the chosen contract is explicit and covered by the relevant documentation and tests, with migration or versioning considered for a breaking change.