Fix webfinger redirect handling in ResolveAccountService (#15187)

Fix webfinger redirect handling in ResolveAccountService (#15187)

* Fix webfinger redirect handling in ResolveAccountService

ResolveAccountService#process_webfinger! handled a one-step webfinger
redirection, but only accepting the result if it matched the exact URI passed
as input, defeating the point of a redirection check.

Instead, use the same logic as in `ActivityPub::FetchRemoteAccountService`,
updating the resulting `acct:` URI with the result of the first webfinger
query.

* Add tests