refactor: rename isAction to isPossibleServerAction (#77011)
this is a pure rename, with no behavioral changes.
`getServerActionMetadata()` only checks if something:
- is definitely a fetch action (sent from the router, with a 'Next-Action' header set)
- is *potentially* a no-js action (a multipart POST where we need to run decodeAction to check)
so `isAction` is a bit misleading. I've renamed it to `isPossibleServerAction`, and updated related names in other places accordingly.
