fix: server actions should fetch from the router canonicalUrl (#80690)
A test related to navigation + server action handling was frequently
flaking in CI
([x-ref](https://github.com/vercel/next.js/actions/runs/15746992788/job/44386768835#step:34:2634)).
When simulating a 20x browser slowdown, it's fairly easy to reproduce
the failure case as well.
The failure occurs due to a race where the router will invoke the server
action against the page the user was on _before_ the navigation, but
_after_ the navigation action has been processed. As a result, the RSC
payload associated with the target page gets replaced by the RSC payload
returned from the server action. There exists a window of time where
`window.location.pathname` is still referring to the previous URL while
`routerState.canonicalUrl` would have been updated to the new page.
This revealed two things:
- The behavior being tested here only tests the expected behavior some
of the time: when it failed, it meant the action wasn't actually
forwarded (not currently addressed by this PR)
- The router state should be the source of truth when determining the
URL to fetch from
next.js
fc993993 - fix: server actions should fetch from the router canonicalUrl (#80690)
