Expression context in prompt placeholders matches policy context

## What

expression context in prompts: use attributes in prompt placeholders, and the same context is not available in policies vs prompt placeholders.

## Want

- Expression context in prompts: use attributes in prompt placeholders,
- The same context is not available in policies vs prompt placeholders

## Ideas this came from

- [#5125 Using attributes in prompt placeholders](https://github.com/goauthentik/authentik/issues/5125) — In the invite link you can predefine the username for the new user in the attribute section while creating the invite link I do this so I define the username an
- [#15902 access to context data in policies vs in prompt stage placeholder](https://github.com/goauthentik/authentik/issues/15902) — Describe your question/ In the policy expression I can do: str = request.context.get("promptdata").get(

Start by reviewing linked issues #5125 and #15902 to understand the requested prompt-placeholder attributes and the context currently exposed to policies. Trace the prompt placeholder and policy expression entry points, then define done as consistent access to the requested context in both places.

Written by the indexing model from the issue text.