fix: broken completion api and return authentication error when xblock is not public #29633

Job	Run time
collect-and-verify	4m 6s
lms-1(py=3.11,dj=pinned,mongo=7.0)	9m 55s
lms-2(py=3.11,dj=pinned,mongo=7.0)	12m 15s
lms-3(py=3.11,dj=pinned,mongo=7.0)	14m 8s
lms-4(py=3.11,dj=pinned,mongo=7.0)	14m 8s
lms-5(py=3.11,dj=pinned,mongo=7.0)	11m 2s
lms-6(py=3.11,dj=pinned,mongo=7.0)	14m 7s
openedx-1-with-lms(py=3.11,dj=pinned,mongo=7.0)	11m 18s
openedx-2-with-lms(py=3.11,dj=pinned,mongo=7.0)	13m 49s
openedx-1-with-cms(py=3.11,dj=pinned,mongo=7.0)	7m 58s
openedx-2-with-cms(py=3.11,dj=pinned,mongo=7.0)	6m 46s
cms-1(py=3.11,dj=pinned,mongo=7.0)	4m 35s
cms-2(py=3.11,dj=pinned,mongo=7.0)	12m 12s
common-with-lms(py=3.11,dj=pinned,mongo=7.0)	9m 21s
common-with-cms(py=3.11,dj=pinned,mongo=7.0)	5m 28s
xmodule-with-lms(py=3.11,dj=pinned,mongo=7.0)	8m 27s
xmodule-with-cms(py=3.11,dj=pinned,mongo=7.0)	8m 51s
Unit tests successful	2s
coverage	0s
compile-warnings-report	0s
merge-artifacts	0s
	2h 48m 28s

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