Upgrade Go to 1.26 and harden CI checks

[alesr]

Raise the module and workflow versions to Go 1.26, run staticcheck, fix vulnerability scan, and validate template fields with new reflect.TypeFor so checks use the registry type parameter directly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.32%. Comparing base (74f6673) to head (3098be5).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   83.25%   82.32%   -0.94%     
==========================================
  Files           4        4              
  Lines         215      215              
==========================================
- Hits          179      177       -2     
- Misses         24       25       +1     
- Partials       12       13       +1     

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