test(csharp): wait for HTTP readiness in fixture

[avirajkhare00]

Summary

• wait for TCP readiness on 8090 and HTTP readiness on 3000 before creating clients
• use the existing unauthenticated /ping endpoint as the HTTP readiness probe
• reduce CI flakes where the C# fixture can hit HTTP before the API is fully started

Testing

• not run (dotnet is not installed in this environment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.83%. Comparing base (484f98a) to head (40328fb).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3024      +/-   ##
============================================
- Coverage     71.91%   71.83%   -0.09%     
  Complexity      930      930              
============================================
  Files          1124     1117       -7     
  Lines         94010    92574    -1436     
  Branches      71357    70107    -1250     
============================================
- Hits          67608    66496    -1112     
+ Misses        23827    23515     -312     
+ Partials       2575     2563      -12     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.21%)	⬇️

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see 65 files with indirect coverage changes

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