Fix cart requests in the local proxy to avoid 401-Unauthorized errors

[karreiro]

WHY are these changes introduced?

Fix the scenario involving the remove button described by:

• [Bug]: Cart actions via traditional HTTP requests fail with 401: Unauthorized or assume Content-Type: application/json #4488
• [Bug]: Cart events oddities #4510

(the checkout piece is not part of the scope of this PR)

WHAT is this pull request doing?

This PR fixes the 401-Unauthorized errors involving cart requests by proxying them instead of relying on the SFR-remote-renderer client. The code is very declarative, but I've added some tests to document the intention.

How to test your changes?

• Run shopify theme init
• Add a regular HTMLAnchorElement to remove a cart item <a href="{{ item.url_to_remove }}">Remove</a> (as described by #4488)
• Remove an element from the cart
• Notice it works as expected

Before

After

Post-release steps

N/A

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

Thanks for your contribution!

Depending on what you are working on, you may want to request a review from a Shopify team:

• Themes: @shopify/advanced-edits
• UI extensions: @shopify/ui-extensions-cli
  • Checkout UI extensions: @shopify/checkout-ui-extensions-api-stewardship
• Hydrogen: @shopify/hydrogen
• Other: @shopify/app-inner-loop

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	72.61% (+0.01% 🔼)	8522/11737
🟡	Branches	69.54% (+0.02% 🔼)	4178/6008
🟡	Functions	71.61%	2205/3079
🟡	Lines	72.92% (+0.01% 🔼)	8064/11058

Test suite run success

1939 tests passing in 873 suites.

Report generated by 🧪jest coverage report action from 41bfd22

[frandiox]

Thanks for the fix and the tests! The other related fix to carts (#4497) was already released a few days ago 👋