fix: Disallow using unconnected accounts in MultichainRouter

[FrederikBolding]

Disallow using unconnected account Snaps for address resolution in MultichainRouter, additionally ensure requests are only routed to connected accounts following resolution.

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Note

Medium Risk
Medium risk because it changes account selection/routing logic for signing requests, which can affect which account is used or cause new request rejections if connectedAddresses is incorrect or missing.

Overview
Prevents MultichainRouter from routing account-Snap signing requests to accounts that are not in the request’s connectedAddresses, including during address resolution.

#getSnapAccountId now filters candidate accounts to those connected first, uses a connected Snap for resolution, and selects only from the connected subset (otherwise throws invalidParams with "No available account found for request."). A new test covers the failure case when no connected accounts are provided.

^{Written by Cursor Bugbot for commit 1eaf930. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (07d1937) to head (1eaf930).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3837   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         430      430           
  Lines       12457    12461    +4     
  Branches     1932     1933    +1     
=======================================
+ Hits        12257    12261    +4     
  Misses        200      200           

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[danroc]

LGTM