[admin/api] Make batch user creation organization field readonly #609

[Eeshu-Yadav]

Description

This PR makes the organization field readonly for existing RadiusBatch objects in both the admin interface and REST API, addressing issue #609.

When the batch user creation object has been created, there's no point in changing the organization field, and this change disallows modification to maintain data integrity.

Changes

Admin Interface

• Modified RadiusBatchAdmin.get_readonly_fields() to include organization field for existing objects
• Organization field remains editable when creating new RadiusBatch objects
• Added comprehensive tests to verify readonly behavior

REST API

• Created RadiusBatchUpdateSerializer with organization as readonly field
• Added BatchUpdateView for handling batch detail operations (retrieve/update)
• Added URL pattern for batch detail endpoint (/radius/batch/<uuid:pk>/)
• Added API tests to verify organization field cannot be modified via API

Testing

• [x] Admin tests: test_radiusbatch_organization_readonly_for_existing_objects
• [x] Admin tests: test_radiusbatch_organization_editable_for_new_objects
• [x] API tests: test_batch_update_organization_readonly
• [x] API tests: test_batch_retrieve_and_update_api
• [x] API tests: test_batch_update_permissions

Checklist

• [x] The code follows the project's coding standards
• [x] Tests have been added/updated to cover the changes
• [x] All tests are passing
• [x] The change addresses the issue requirements
• [x] Backward compatibility is maintained (organization field still editable for new objects)

Fixes #609

[Eeshu-Yadav]

@pandafy kindly review this

[nemesifier]

@Eeshu-Yadav what's the point of reviewing if you don't follow up?

Please follow up with https://github.com/openwisp/openwisp-radius/pull/621.

[Eeshu-Yadav]

@Eeshu-Yadav what's the point of reviewing if you don't follow up?

Please follow up with #621.

ok