fix: Prevent concurrent wallet_requestSnaps requests

[FrederikBolding]

Adds a mutex to the wallet_requestSnaps handler to prevent race conditions if multiple concurrent requests are sent from the same origin. Since Snaps are defined as caveats we need to merge permissions and for that we need the most up-to-date permissions-set, without this change we would be merging with stale data.

Recommend reviewing this PR with whitespace diff hidden.

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Note

Prevents concurrent processing of wallet_requestSnaps that could race when merging permissions.

• Introduces a per-origin Mutex in requestSnaps.ts and wraps the permission/install flow in runExclusive, using the request origin to serialize operations
• Moves empty-params validation outside the try/catch for earlier exit; preserves error handling by setting res.error within the mutex block
• Slightly updates coverage thresholds in jest.config.js

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (15a9666) to head (0e1386e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3800   +/-   ##
=======================================
  Coverage   98.33%   98.33%           
=======================================
  Files         422      422           
  Lines       12073    12082    +9     
  Branches     1875     1876    +1     
=======================================
+ Hits        11872    11881    +9     
  Misses        201      201           

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