sources/scim: fix user creation (duplicate userName)

[sia-mfierro]

Details

According to the SCIM standard both id and userName must be unique.
The current behaviour only rejects duplicate ids, and this PR fixes it.
A new test case has been added: test_user_create_duplicate_by_username, and run.
Results from https://scimvalidator.microsoft.com/ have been saved and replayed for local testing.

---

Checklist

• Local tests pass (sudo poetry run python -m manage test authentik/sources/scim/tests/test_users.py)
• The code has been formatted (make lint-fix)

[netlify]

✅ Deploy Preview for authentik-storybook canceled.

Name	Link
🔨 Latest commit	eab1f0e
🔍 Latest deploy log	https://app.netlify.com/sites/authentik-storybook/deploys/67778dc9167d210008d1a54b

[netlify]

✅ Deploy Preview for authentik-docs canceled.

Name	Link
🔨 Latest commit	eab1f0e
🔍 Latest deploy log	https://app.netlify.com/sites/authentik-docs/deploys/67778dc98b3c7e00084bf0fb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (bdf4236) to head (eab1f0e).
Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12547      +/-   ##
==========================================
+ Coverage   92.69%   92.79%   +0.09%     
==========================================
  Files         770      770              
  Lines       38873    38885      +12     
==========================================
+ Hits        36033    36082      +49     
+ Misses       2840     2803      -37     

Flag	Coverage Δ
e2e	48.74% <7.69%> (+0.14%)	⬆️
integration	24.57% <0.00%> (-0.01%)	⬇️
unit	90.41% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.