[14.0][IMP] connector_extension: add user_id field and work_on override

[eantones]

Summary

• Add optional user_id field on the connector backend to configure a default user for all sync operations
• Override work_on to switch to this user before creating the WorkContext, ensuring all components, record creation, and child jobs (including delayed ones) run under the configured user context
• Prevents records from being created under the session user when imports are triggered manually from the backend form

Test plan

• Set user_id on a backend, trigger import manually — verify records are created under the configured user
• Leave user_id empty — verify behavior is unchanged (records created under session user)
• Trigger import via cron — verify the backend's user_id overrides the cron's user

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.53%. Comparing base (ce814c2) to head (a04f67c).
⚠️ Report is 1 commits behind head on 14.0.

Files with missing lines	Patch %	Lines
connector_extension/models/backend/backend.py	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             14.0     #871      +/-   ##
==========================================
- Coverage   50.53%   50.53%   -0.01%     
==========================================
  Files         979      979              
  Lines       16685    16693       +8     
  Branches     3558     3559       +1     
==========================================
+ Hits         8432     8436       +4     
- Misses       8034     8039       +5     
+ Partials      219      218       -1     

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