test: detect available python command in wpt runner

[mcollina]

Summary

• detect whether python3 or python is available before invoking WPT tooling
• reuse the discovered Python command for setup, manifest generation, host file generation, and the WPT server
• fail with a clearer error when no usable Python 3 command is available

Testing

• CI=true npm run test:wpt:setup
• CI=true npm run test:wpt

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.36%. Comparing base (363e44f) to head (6576f16).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5427   +/-   ##
=======================================
  Coverage   93.35%   93.36%           
=======================================
  Files         110      110           
  Lines       36993    36993           
=======================================
+ Hits        34535    34537    +2     
+ Misses       2458     2456    -2     

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