Refresh entity list after updating custom group (self_hook_*)

[totten]

Overview

The CustomGroups are used to generate virtual entities (if they set is_multiple). Toggling is_multiple
should cause the virtual entity to be added or removed.

This is an alternative variant of #22843.

Reproduction steps

1. Create a custom data group. Leave default is_multiple=0.
2. View "API Explorer". Look for the virtual entity. (It's not there. Properly so.)
3. Edit custom data group. Change to is_multiple=1.
4. Reload "API Explorer". Look for the virtual entity.

Before

The entity does not appear. You have to do a system-flush.

After

The entity does appear.

[civibot]

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[totten]

Merging based on @colemanw sign-off in the discussion of #22843. (The two are basically the same - and the discussion about trade-offs is in the other.)

This has test-coverage and addresses an alpha regression, so it's easier to merge before freezing for beta.