feat: Add NPM proxy support

[FrederikBolding]

Add a useNpmProxy flag to the NPM location options. When enabled, this proxy URL is used instead of the main NPM registry URL. The implementation assumes that the proxy returns identical content and respects the same headers.

This flag is enabled only for updating preinstalled Snaps.

---

Note

Adds a useNpmProxy option to NPM locations and routes automatic preinstalled Snap updates through the MetaMask NPM proxy.

• Snaps Controller (packages/snaps-controllers/src/snaps/SnapController.ts):
  • Use detectSnapLocation(..., { useNpmProxy: true }) when auto-updating preinstalled Snaps during updateRegistry.
• NPM Location (packages/snaps-controllers/src/snaps/location/npm.ts):
  • Add NPM_REGISTRY_PROXY and new NpmOptions.useNpmProxy flag.
  • Default registry selection now uses proxy when useNpmProxy is true.
  • Update isNPM to treat proxy URL as NPM.

^{Written by Cursor Bugbot for commit 4dda778. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (0b412ce) to head (4dda778).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3695   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files         418      418           
  Lines       12146    12150    +4     
  Branches     1875     1878    +3     
=======================================
+ Hits        11937    11941    +4     
  Misses        209      209           

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