fix: Ensure iframe load listener is removed

[FrederikBolding]

Ensure the load listener for the iframe is removed after the promise resolves. Found while investigating a memory leak, but does not fix it.

---

Note

Ensures the iframe load handler is one-time and self-cleaning.

• Updates iframe.ts to use addEventListener('load', handler, { once: true }), so the listener automatically removes itself after resolving/rejecting, preventing duplicate fires and lingering listeners

^{Written by Cursor Bugbot for commit 56dce3b. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.34%. Comparing base (6653139) to head (56dce3b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/snaps-utils/src/iframe.ts	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3797   +/-   ##
=======================================
  Coverage   98.34%   98.34%           
=======================================
  Files         422      422           
  Lines       12059    12060    +1     
  Branches     1872     1872           
=======================================
+ Hits        11859    11860    +1     
  Misses        200      200           

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