feat: Allow mocking JSON-RPC implementations

[FrederikBolding]

Allow mocking full JSON-RPC method implementations by passing functions. Also introduces mockJsonRpcOnce, which only mocks the first call to the method. The previous method of specifying mocks is still supported.

Closes #2096

---

Note

Adds function-based JSON-RPC mocking and a new mockJsonRpcOnce helper (with queueing), refactoring store/middleware to support per-mock implementations and one-time behavior.

• API:
  • Extend mockJsonRpc to accept function implementations in packages/snaps-simulation/src/helpers.ts and types in packages/snaps-simulation/src/types.ts.
  • Add mockJsonRpcOnce to Snap/helpers and expose via packages/snaps-jest/src/helpers.ts.
• Store & Middleware:
  • Refactor mocks state to store { implementation, once } by id; remove getJsonRpcMock, keep getJsonRpcMocks (packages/snaps-simulation/src/store/mocks.ts).
  • Update middleware to async, iterate mocks, execute implementations, and auto-remove once mocks (packages/snaps-simulation/src/middleware/mock.ts).
  • Introduce nanoid-based IDs and allow non-serializable middleware (serializableCheck: false).
• Tests:
  • Add tests for function-based mocks, mockJsonRpcOnce, and queued mocks in helpers tests; update middleware/store tests to new structure.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (3754d30) to head (5c765f1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3667   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files         417      417           
  Lines       11907    11924   +17     
  Branches     1848     1852    +4     
=======================================
+ Hits        11704    11721   +17     
  Misses        203      203           

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