refactor: Use hash-private in BaseSnapExecutor

[FrederikBolding]

This PR refactors BaseSnapExecutor to follow our more recent patterns, notably using hash private for most properties and functions, but also changing a couple of naming patterns and improving types. This makes BaseSnapExecutor way more compliant with our linter.

Split off #3803 to make this easier to review.

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Note

Medium Risk
Touches core snap execution/termination and teardown tracking; while mostly a refactor, it changes how termination promises and teardown state are managed and could affect edge-case lifecycle behavior.

Overview
Refactors BaseSnapExecutor to use hash-private fields/methods throughout (removing prior lint suppressions) and tightens command dispatch typing by calling #methods[method as keyof Methods] instead of any.

Updates execution/termination plumbing: snap evaluation cancellation now uses createDeferredPromise and a shared #teardownRef object is passed into withTeardown instead of this, changing the termination error string to The Snap "..." has been terminated during execution. (and updating the corresponding browser test expectation).

^{Written by Cursor Bugbot for commit 0082a8c. This will update automatically on new commits. Configure here.}

[FrederikBolding]

This typing is still not great. The ideal case is to make a breaking change to always pass parameters as objects and use structs to validate them better

[codecov]

Codecov Report

❌ Patch coverage is 97.26027% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.39%. Comparing base (5ab30ee) to head (0082a8c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...cution-environments/src/common/BaseSnapExecutor.ts	97.26%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3830      +/-   ##
==========================================
- Coverage   98.40%   98.39%   -0.01%     
==========================================
  Files         430      430              
  Lines       12448    12457       +9     
  Branches     1933     1932       -1     
==========================================
+ Hits        12249    12257       +8     
- Misses        199      200       +1     

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