Fix DTO code generation for C# static proxy

[realLiangshiwei]

Description

Resolves #19137

Checklist

• I fully tested it as developer / designer and created unit / integration tests
• I documented it (or no need to document or I will create a separate documentation issue)

How to test it?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 51.54%. Comparing base (67db1ab) to head (879e08b).
Report is 1 commits behind head on rel-8.0.

Files	Patch %	Lines
...viceProxying/CSharp/CSharpServiceProxyGenerator.cs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           rel-8.0   #19145      +/-   ##
===========================================
- Coverage    51.54%   51.54%   -0.01%     
===========================================
  Files         3080     3080              
  Lines        97400    97405       +5     
  Branches      7774     7775       +1     
===========================================
  Hits         50209    50209              
- Misses       45640    45645       +5     
  Partials      1551     1551              

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