feat: add pluggable override for learner home enterprise customer lookup (#2554)

* feat: add pluggable override for learner home enterprise customer lookup

ENT-11571

* chore: quality fixes

* docs: OVERRIDE_LEARNER_HOME_GET_ENTERPRISE_CUSTOMER TODO

* chore: bump version

* docs: Add type contract for get_enterprise_customer

* chore: remove prev_fn fallback

* chore: put OVERRIDE_LEARNER_HOME_GET_ENTERPRISE_CUSTOMER in settings

* test: Remove unneeded _call

* test: Organize patching mocked methods

* chore: fix import issues

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Co-authored-by: Marlon Keating <mkeating@2u.com>, Claude