fix: require course run for late custom enrollments

[zwidekalanga]

Description:
Late enrollment was still depending on course-level metadata resolving to an advertised or future run. That blocks custom/one-off presentations where the intended run is already past and no future cohort exists.

This PR makes the assignment path fail early with an actionable error when a course key does not resolve to a concrete course run, and updates operator-facing guidance to use exact course run keys for custom late enrollments. It also keeps the forced-redemption assignment path covered so the requested run remains the assignment's preferred run.

What changed

• Require assignment metadata to include a resolved course_run_key before creating/updating assignments.
• Return the exact error text to API callers instead of the generic allocation message.
• Clarify allocation API/admin help text for custom one-off presentations.
• Add the investigation and implementation plan in docs/late-enrollment-custom-presentations-plan.md.

Related PRs

• Service-side redemption guard: fix: validate late enrollment course run metadata openedx/enterprise-subsidy#436

Testing

• pytest enterprise_access/apps/content_assignments/tests/test_api.py::TestAssignmentAllocationAndCancellation::test_allocate_assignments_requires_resolved_course_run enterprise_access/apps/subsidy_access_policy/tests/test_forced_redemption.py::ForcedPolicyRedemptionAssignmentTests::test_force_redemption_with_assignment_happy_path
• python -m py_compile enterprise_access/apps/content_assignments/api.py enterprise_access/apps/api/serializers/subsidy_access_policy.py enterprise_access/apps/content_assignments/tests/test_api.py enterprise_access/apps/subsidy_access_policy/tests/test_forced_redemption.py
• git diff --check

Jira:
ENT-XXXX

Merge checklist:

• ./manage.py makemigrations was not required; no model fields were changed.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.13%. Comparing base (409cec2) to head (b27e27a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #184   +/-   ##
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  Coverage   86.13%   86.13%           
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  Files         151      151           
  Lines       12547    12554    +7     
  Branches     1197     1198    +1     
=======================================
+ Hits        10807    10814    +7     
  Misses       1425     1425           
  Partials      315      315           

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