Blueprint delete of stage bindings is unsafe when KeyOf is still referenced

## What

blueprint delete of stage bindings is unsafe (!KeyOf still referenced; deleting a missing binding fails the whole apply).

## Want

- Blueprint delete of stage bindings is unsafe (!KeyOf still referenced
- Deleting a missing binding fails the whole apply)

## Ideas this came from

- [#20309 Blueprints fail to delete stage when it references another stage using !KeyOf](https://github.com/goauthentik/authentik/issues/20309) — I am trying to add a delete option for a blueprint, so it can be cleanly uninstalled with "one click". However, even though the blueprint execution is "successf
- [#20456 Deleting non-existent flow stage bindings via blueprints fails](https://github.com/goauthentik/authentik/issues/20456) — Trying to delete a flow stage binding from a flow fails if the binding is already deleted. Consider the following blueprint Applying this blueprting when the fl

Start by reproducing the two blueprint deletion cases described in this issue and compare them with linked issues #20309 and #20456. Done means deleting stage bindings remains safe when !KeyOf references another stage, and an already-missing binding does not fail the whole blueprint apply.

Written by the indexing model from the issue text.