Enforce export compliance verification on course/program enrollment

[annagav]

What are the relevant tickets?

Related to #3706

Description (What does it do?)

• Wires the compliance app's CyberSource export check into create_run_enrollments and create_program_enrollments via a new _verify_exports_compliance_for_enrollment helper in courses/api.py, gated
  behind the EXPORT_COMPLIANCE_CHECK_ENABLED feature flag (main/features.py)
• Enrollment is blocked and an ExportComplianceError is raised when CyberSource does not accept the check, for both course-run and program enrollments (audit and verified modes)
• Surfaces ExportComplianceCheckError as a user-facing error from the v1–v3 enrollment surfaces:
  • courses/serializers/v1/v2/v3/courses.py — raises a DRF ValidationError with exc.to_error_detail()
  • courses/views/v1/init.py — redirects with a user-facing enrollment-blocked message
  • courses/views/v2/init.py — returns a 400 response, including for the verified-program-enrollment reconciliation path
  • courses/views/v3/init.py — raises a ValidationError
• Adds an autouse mocked_export_compliance conftest fixture so existing tests continue to pass with the check enabled by default, and updates drf_lint_baseline.json for line shifts in the touched
  serializers
• Updates _verify_exports_compliance_for_enrollment to pass a courseware object (a CourseRun for run enrollments, a Program for each program enrollment) through to verify_user_with_exports, matching that function's (user, courseware_object) signature from Add CyberSource export compliance data model #3793

Stacked on nl/export-compliance-cache (#3793), which adds the ExportComplianceLog caching model this PR's compliance checks read/write through, and which is itself stacked on ag/export-compliance-app (#3785). This PR only contains the enrollment integration; diff against the base branch reflects just that.

Test plan

• docker compose exec web pytest courses/api_test.py
• docker compose exec web pytest courses/
• Manually exercise course-run and program enrollment with EXPORT_COMPLIANCE_CHECK_ENABLED=True for both an accepted and a rejected CyberSource decision

[github-actions]

OpenAPI Changes

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