refactor(app): extract handler returned response handling

[pi0]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This is an internal refactor to make future implementation changes easier for handling values directly returned in event handler as response.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #473 (db94d67) into main (06d1bc1) will increase coverage by 0.07%.
The diff coverage is 94.91%.

@@            Coverage Diff             @@
##             main     #473      +/-   ##
==========================================
+ Coverage   79.77%   79.85%   +0.07%     
==========================================
  Files          27       27              
  Lines        2987     2993       +6     
  Branches      441      445       +4     
==========================================
+ Hits         2383     2390       +7     
+ Misses        604      603       -1     

Files Changed	Coverage Δ
src/app.ts	96.52% <94.91%> (-1.25%)	⬇️

... and 1 file with indirect coverage changes