fix: enroll with the open mode the course offers instead of hardcoded audit

[ccantillo]

Fix "Start Course" enrollment failure — openedx-corporate

Problem

Clicking Start Course on a catalog course card showed "Enrollment failed"
for users not yet enrolled in the course. The card already calls
POST /partner_catalog/api/v1/catalogs/{id}/courses/{course_id}/enroll/, but
for open courses the backend always requested the LMS enrollment with a
hardcoded audit mode. NAU courses are configured with honor instead, so
the LMS raised CourseModeNotFoundError (an unhandled 500).

This is also why the button "started working" after enrolling through the
course page: the learning MFE enrolls with the course's default mode, and once
any LMS enrollment exists the failing add_enrollment call is skipped.

What changed

partner_catalog/services/platform_enrollment.py

• ensure_edx_platform_enrollment now resolves open-access targets to the
  open mode the course actually offers (audit preferred, honor fallback)
  instead of passing audit blindly.
• An existing open-mode enrollment satisfies an open-access target without an
  unnecessary update_enrollment call.
• The function returns the effective LMS mode. This also fixes the same
  latent crash in downgrade_to_audit (unenrollment) for honor-mode courses.

partner_catalog/services/enrollments.py

• _sync_lms_for_catalog_access reports the effective mode returned by the
  sync instead of assuming the hardcoded target, so the API response's
  lms_enrollment_mode is accurate.