Refactor stub_asset_proxy_setting to be less error-prone

Asherah Connorrequested to merge
push-xzkxunpnqosz into master

What does this MR do and why?

Fixes Refactor stub_asset_proxy_setting uses in specs... (#582610 - closed) by reusing the live logic so we don't have to keep the application and stub in sync manually.

How to set up and validate locally

Spec-only change — run affected specs, see that they still pass. CI is probably good enough.

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