refactor: Include test files in type checking

[y-hsgw]

I noticed several type errors in the tests directory, so I updated the config to include them in type-checking.
Let me know what you think!

Checks

• I have read the contributing guidelines.

Changes

• Created a separate tsconfig.build.json for build purposes.
  → To type-check both lib and tests, but exclude tests from the build output.
• Fixed type errors that appeared during type-check.

Context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (36facf1) to head (350bcd3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1022   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files          47       47           
  Lines        2534     2535    +1     
  Branches     1043     1043           
=======================================
+ Hits         2440     2441    +1     
  Misses         94       94           

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[Belco90]

I wondered myself many times if we should enable this, but never had the time to do it. Thanks!