chore: remove dead config->map decode and unused mapstructure import

[Forostovec]

This change removes an unnecessary config-to-map mapstructure.Decode in node.Node.Run and the corresponding unused mapstructure import. The decoded map was never used (logging uses yaml.Marshal(n.cfg) directly), which made the block dead code and added avoidable allocations and an extra failure path during startup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.64%. Comparing base (fe20d83) to head (ed90591).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3250      +/-   ##
==========================================
- Coverage   76.68%   76.64%   -0.05%     
==========================================
  Files         322      322              
  Lines       31841    31842       +1     
==========================================
- Hits        24417    24404      -13     
+ Misses       5685     5682       -3     
- Partials     1739     1756      +17     

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