re-add hello app host

[rysweet]

the sun file got munched in a conflict resolution - removed it and readied it.

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.16%. Comparing base (edbd201) to head (7dad735).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5445   +/-   ##
=======================================
  Coverage   76.16%   76.16%           
=======================================
  Files         196      196           
  Lines       10672    10672           
  Branches      139      139           
=======================================
  Hits         8128     8128           
  Misses       2449     2449           
  Partials       95       95           

Flag	Coverage Δ
unittests	76.16% <ø> (ø)

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