chore!: Remove snap_getCurrencyRate

[FrederikBolding]

Remove snap_getCurrencyRate as it was an experimental never-documented API used for the BTC Snap.

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Note

Medium Risk
Breaking API removal that will impact any Snap calling snap_getCurrencyRate and any consumers relying on the removed SDK types, but changes are largely deletions with minimal logic risk.

Overview
Removes the experimental snap_getCurrencyRate JSON-RPC method end-to-end, including its permitted handler (getCurrencyRate.ts), tests, and registration in snaps-rpc-methods.

Updates snaps-sdk type exports by deleting the get-currency-rate method types and removing snap_getCurrencyRate from the SnapMethods map, and slightly adjusts Jest coverage thresholds to reflect the deletion.

^{Written by Cursor Bugbot for commit a9928ef. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (5c57821) to head (a9928ef).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3866      +/-   ##
==========================================
- Coverage   98.58%   98.57%   -0.01%     
==========================================
  Files         426      425       -1     
  Lines       12332    12311      -21     
  Branches     1921     1919       -2     
==========================================
- Hits        12157    12136      -21     
  Misses        175      175              

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