stages/prompt: set allow_blank for _read_only fields (cherry-pick #18297 to version-2025.10)

[authentik-automation]

Cherry-pick of #18297 to version-2025.10 branch.

Original PR: #18297
Original Author: @BeryJu
Cherry-picked commit: d5f730a

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.72%. Comparing base (f80ce9d) to head (abb3913).
⚠️ Report is 2 commits behind head on version-2025.10.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
authentik/stages/prompt/models.py	50.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (f80ce9d) and HEAD (abb3913). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (f80ce9d)	HEAD (abb3913)
unit	10	8
unit-migrate	10	3

Additional details and impacted files

@@                 Coverage Diff                 @@
##           version-2025.10   #18406      +/-   ##
===================================================
- Coverage            92.93%   87.72%   -5.21%     
===================================================
  Files                  869      869              
  Lines                48024    48025       +1     
===================================================
- Hits                 44632    42131    -2501     
- Misses                3392     5894    +2502     

Flag	Coverage Δ
e2e	45.18% <50.00%> (+0.05%)	⬆️
integration	23.15% <50.00%> (-0.01%)	⬇️
unit	85.42% <50.00%> (-5.67%)	⬇️
unit-migrate	64.06% <50.00%> (-27.07%)	⬇️

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[github-actions]

authentik PR Installation instructions

Instructions for docker-compose

Add the following block to your .env file:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-abb391369b7c9ebcdc5ba842cbaec1ea1eaf01fc
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

Afterwards, run the upgrade commands from the latest release notes.

Instructions for Kubernetes

Add the following block to your values.yml file:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
global:
    image:
        repository: ghcr.io/goauthentik/dev-server
        tag: gh-abb391369b7c9ebcdc5ba842cbaec1ea1eaf01fc

Afterwards, run the upgrade commands from the latest release notes.