feat: block AccountTreeController messenger calls

[ccharly]

We'll soon introduce some export/import feature on the AccountTreeController, so it's best if we block it from messenger calls for now.

---

Note

Low Risk
Single-line addition to an existing Snap RPC denylist; tightens access rather than widening it.

Overview
AccountTreeController is added to the denylist used by snap_messengerCall, so preinstalled Snaps can no longer dispatch messenger actions whose names start with that client (same pattern as other blocked controllers).

This is a proactive lockdown ahead of planned account-tree export/import work, reducing the chance Snaps reach that surface before it is intentionally exposed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (14eeb53) to head (aa86561).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4100   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         429      429           
  Lines       12495    12495           
  Branches     1976     1976           
=======================================
  Hits        12320    12320           
  Misses        175      175           

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