fix: Make KeyringOrigin caveat explicitly optional

[FrederikBolding]

Unintentional or not, endowment:keyring has since its implementation allowed no caveats to be passed until 79e8b90. This was caused by the caveat mapper constructing the caveat with an empty object as the value, which is valid according to our validation: https://github.com/MetaMask/snaps-skunkworks/blob/fb/keyring-origins-optional/packages/snaps-utils/src/json-rpc.ts#L77

The Bitcoin Snap used in production specifies endowment:keyring: {} in its manifest.

This PR makes the permission validation explicit in making KeyringOrigin optional, including adding a fallback in getKeyringCaveatOrigins.

---

Note

Medium Risk
Adjusts permission validation and defaults for the security-adjacent endowment:keyring caveats; incorrect optionality or fallback behavior could inadvertently widen allowed origins if downstream assumes the caveat is always present.

Overview
Makes endowment:keyring permission validation explicitly treat the keyringOrigin caveat as optional (matching other keyring caveats), rather than requiring it to be present.

Updates getKeyringCaveatOrigins to gracefully default to { allowedOrigins: [] } when the caveat (or caveats list) is missing, and aligns tests/coverage thresholds with the new behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.14%. Comparing base (c339e31) to head (5ca964e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3955      +/-   ##
==========================================
+ Coverage   98.56%   99.14%   +0.57%     
==========================================
  Files         428      346      -82     
  Lines       12348     9314    -3034     
  Branches     1939     1539     -400     
==========================================
- Hits        12171     9234    -2937     
+ Misses        177       80      -97     

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