providers/oauth2: evaluate property mappings in client credentials JWT flow

[Rishabh-git10]

Details

Currently, the client_credentials flow using JWT assertions bypasses property mappings, hardcoding the generated user's attributes based solely on the sub claim and application name.

This PR updates __create_user_from_jwt to leverage the core SourceMapper engine. It now properly evaluates configured user property mappings and applies them to the generated service account, aligning this flow's behavior with standard Authentik logins.

• Replaced manual user attribute assignment with SourceMapper().build_object_properties().
• Added unit tests to verify correct mapping extraction from the incoming JWT payload.

closes #18767

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make docs)

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[BeryJu]

Looks good, made a couple small changes:

• Always top level imports unless they're needed inline to avoid circular imports
• clamp the username at its max length (technically an unrelated change)
• keep the original test case and change your modification into a new test case

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (9584cee) to head (c8aa8d1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20979      +/-   ##
==========================================
- Coverage   93.49%   93.44%   -0.06%     
==========================================
  Files         993      993              
  Lines       56126    56146      +20     
==========================================
- Hits        52477    52467      -10     
- Misses       3649     3679      +30     

Flag	Coverage Δ
conformance	37.39% <8.33%> (-0.01%)	⬇️
e2e	42.88% <8.33%> (-0.03%)	⬇️
integration	22.16% <0.00%> (-0.06%)	⬇️
unit	91.69% <100.00%> (+<0.01%)	⬆️
unit-migrate	91.79% <100.00%> (+<0.01%)	⬆️

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