fix: properly avoid race condition when provisioning licenses

[iloveagent57]

ENT-10064
We should have been using transaction.on_commit() here all along.
https://docs.djangoproject.com/en/5.1/topics/db/transactions/#performing-actions-after-commit

Link to the associated ticket: https://openedx.atlassian.net/browse/ENT-10064

Testing considerations

1. Tail the worker logs make worker-logs.
2. Create a new subs plan with say, 20k licenses. Save it.
3. Observe in the worker logs that the worker asynchronously provisions license records for the plan.

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

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[iloveagent57]

Moved to a TestCase class so I can utilize this on_commit test utility to actually execute the provision_licenses() function.