Terminate the enrollment error code sentence with a period

[rhysyngsun]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/13250

Description (What does it do?)

Follow-up to #3938, from review feedback on
mitodl/mit-learn#3918 (comment). The
appended support code left the detail unterminated - "Unable to complete enrollment. Error code: CS_700" - and MIT Learn renders a server detail
followed by its own support copy, so it read as a run-on: "... Error code: CS_700 Contact Support for assistance."

• EnrollmentError.from_cause now ends the appended sentence with a period.
• The no-code path is unchanged; default_detail already ended in one.

How can this be tested?

Tests should pass. I ran, on this branch:

• docker compose run --rm web pytest -n0 courses/exceptions_test.py (3 passed)
• docker compose run --rm web pytest -n0 --no-cov courses/views/v1/views_test.py courses/views/v2/views_test.py courses/views/v3/views_test.py -k export_compliance (10 passed)

Seeing the rendered copy needs mitodl/mit-learn#3918;
I have not re-run that end to end.

Additional Context

The review offered three fixes - wrap the copy onto multiple lines, detect the
missing period client-side, or terminate it upstream. This is the upstream one,
so Learn needs no change. Learn still renders any 400 detail verbatim into
that sentence, so an unterminated detail from anywhere else would reproduce it.

[github-actions]

OpenAPI Changes

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Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[asadali145]

LGTM!