fix: Handle withKeyring breaking change in MultichainRouter

[FrederikBolding]

KeyringController.withKeyring had a breaking change in MetaMask/core@8e15f01 where the format of the callback parameters was changed. This breaks the SIP-26 branch as the extension is already using the latest version of the KeyringController. This PR adjusts our usage of withKeyring to expect the new format.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.93%. Comparing base (adecea5) to head (6b6f171).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3238   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         508      508           
  Lines       11219    11219           
  Branches     1730     1730           
=======================================
  Hits        10651    10651           
  Misses        568      568           

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