[#2587] Require all services except forms in ZGWApiGroupConfig

[swrichards]

Note that the changes in the test files were necessary because the OpenZaakConfig legacy getter/setter methods relied on most services being nullable, enabling it to dynamically create each missing service as it was set. That is obviously no longer possible, so now you'll see a lot of:

ZGWApiGroupConfigFactory(
  ztc_service__api_root=CATALOGI_ROOT,
  zrc_service__api_root=ZAKEN_ROOT,
  form_service=None,
)

in place of

self.config = OpenZaakConfig.get_solo()
self.config.zaak_service = ServiceFactory(api_root=ZAKEN_ROOT, api_type=APITypes.zrc)
self.config.catalogi_service = ServiceFactory(api_root=CATALOGI_ROOT, api_type=APITypes.ztc)
self.config.save()       

This also functions as a general cleanup to pave the way for the coming changes: the test setups would've had to evolve eventually anyway, and this will make the logic around multiple backends a bit easier.

Taiga: #2587

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.46835% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.14%. Comparing base (4b02eb4) to head (3b4c573).

Files	Patch %	Lines
src/open_inwoner/cms/cases/tests/test_htmx.py	0.00%	1 Missing ⚠️
src/open_inwoner/openzaak/models.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1278      +/-   ##
===========================================
- Coverage    95.17%   95.14%   -0.04%     
===========================================
  Files          981      982       +1     
  Lines        35665    35626      -39     
===========================================
- Hits         33945    33896      -49     
- Misses        1720     1730      +10     

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[alextreme]

Good cleanup and helps to avoid inconsistent configuration