Validate method existence and standardize error handling

[BelfordZ]

Summary

• ensure router verifies method implementations before invocation
• replace non-standard error code with JSON-RPC internal error
• add tests for unimplemented and internal error cases

Testing

• npm test
• npm run lint

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https://chatgpt.com/codex/tasks/task_e_68b33c4ff1c4832f9345bdb4a36860cf

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (eee4a4d) to head (8e6df61).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
+ Coverage   98.96%   98.97%   +0.01%     
==========================================
  Files           9        9              
  Lines         289      292       +3     
  Branches       43       45       +2     
==========================================
+ Hits          286      289       +3     
  Misses          3        3              

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