perf: Unload iframe before removing

[FrederikBolding]

To drastically reduce the amount of memory leaked if the iframe cannot be garbage collected properly, this PR modifies terminateJob to navigate the iframe away from the Snaps execution environment before removing it, effectively unloading it.

Before:

Explicit Allocations

15.36 MB (100.0%) -- explicit
└──15.36 MB (100.0%) -- window-objects/top(none)/detached
   ├──15.26 MB (99.40%) -- window(http://localhost:6363/)
   │  ├───7.67 MB (49.97%) ++ js-realm(moz-nullprincipal:{45eed705-e0da-4e2b-b0bf-e7ac336c93f4}?http://localhost:6363, http://localhost:6363/)
   │  ├───5.76 MB (37.49%) ++ js-realm(moz-nullprincipal:{8cb3b86e-774f-4a1a-bf6a-eeac2636fe9f}?http://localhost:6363, http://localhost:6363/)
   │  ├───1.83 MB (11.94%) ++ dom
   │  └───0.00 MB (00.00%) ++ (2 tiny)
   └───0.09 MB (00.60%) ++ window(chrome://devtools/content/shared/webextension-fallback.html#webextension-flask@metamask.io)

After:

Explicit Allocations

0.14 MB (100.0%) -- explicit
└──0.14 MB (100.0%) -- window-objects/top(none)/detached/window(about:blank)
   ├──0.06 MB (46.17%) ++ js-realm(moz-nullprincipal:{0114dc8e-3f5e-4a2c-8008-c4219c84dcc0}?moz-extension://1465affa-73bf-4b77-9103-8afb21b2fa9d, about:blank)
   ├──0.06 MB (46.17%) ++ js-realm(moz-nullprincipal:{7e2fd14e-3040-493d-862a-9c207eae5213}?moz-extension://1465affa-73bf-4b77-9103-8afb21b2fa9d, about:blank)
   ├──0.01 MB (07.16%) ++ dom
   └──0.00 MB (00.50%) ++ (2 tiny)

---

Note

Improves termination flow and reduces iframe memory retention by making job termination asynchronous and explicitly unloading iframes before removal.

• Changes AbstractExecutionService.terminateJob to return a Promise<void> and awaits it in terminateSnap and when init times out in #initStreams
• In IframeExecutionService, terminateJob now navigates the iframe to about:blank (with a short withTimeout-guarded wait for load) before removing it
• Updates NodeProcessExecutionService and WebViewExecutionService terminateJob implementations to async (no functional change beyond signature)

^{Written by Cursor Bugbot for commit c5ec278. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.33%. Comparing base (6653139) to head (c5ec278).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...lers/src/services/iframe/IframeExecutionService.ts	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3796      +/-   ##
==========================================
- Coverage   98.34%   98.33%   -0.01%     
==========================================
  Files         422      422              
  Lines       12059    12067       +8     
  Branches     1872     1873       +1     
==========================================
+ Hits        11859    11866       +7     
- Misses        200      201       +1     

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