Repair cart page including reinstated course api improvements

[collinpreston]

What are the relevant tickets?

#1876
#1869

Description (What does it do?)

1. Reinstates: 1869: Course program api performance improvements #1872 & Department REST API #1877
2. Fixes an issue when loading the cart page (https://mitxonline.mit.edu/cart/) with an item in the basket. Issue ref: https://mit-office-of-digital-learning.sentry.io/issues/4472765075/?alert_rule_id=8724694&alert_type=issue&notification_uuid=2bbf21e8-fcf4-46ec-944a-9cc1c91f3c80&project=5864687&referrer=slack

How can this be tested?

1. Enroll into a course and proceed with certificate enrollment.
2. When redirected to the cart page, ensure that the cart page loads as expected and there are no errors in the browser console.
3. Through http://mitxonline.odl.local:8013/cms/, add a flexible price child page to the course page for the course you're enrolled in.
4. Edit the course page and add the URL of the flexible price page to the URL field under the "price details" section.
5. Refresh the cart page and ensure that you see a "Need financial assistance?" link.

Additional Context

This was caused because I was passing a course to isFinancialAssistanceAvailable, here: https://github.com/mitodl/mitxonline/blob/c87bdc15e75c6bae728749dedd5a91cae3755c8d/frontend/public/src/containers/pages/checkout/CartPage.js#L141C15-L141C15

[pdpinch]

Are there no tests for this code? I would have expected this to be caught by a test somewhere.

More forward-thinking, would type checking have helped?

[collinpreston]

Are there no tests for this code? I would have expected this to be caught by a test somewhere.

More forward-thinking, would type checking have helped?

@pdpinch There are no tests for the cart page. Typing existed but did not help here. I can add tests but I don't think it will delay this by a day.

[jkachel]

Functionality looks good to me. I tested by deleting the finaid form for one of my local programs, then adding a form to a course in the program, then deleting that and adding a form back to the program; all tests were successful. 👍