Generate only the types used by service signatures in C# client proxies

[maliming]

CSharpServiceProxyGenerator now generates only contract types reachable from service signatures. It uses API assembly metadata and client project assets to avoid regenerating referenced contract types.

Nested, generic, and same-named contract types are handled in generated DTOs and proxy signatures.

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[codecov]

Codecov Report

❌ Patch coverage is 86.05263% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.30%. Comparing base (c80c6f8) to head (6e4b1cf).
⚠️ Report is 52 commits behind head on dev.

Files with missing lines	Patch %	Lines
...viceProxying/CSharp/CSharpServiceProxyGenerator.cs	86.20%	34 Missing and 18 partials ⚠️
...c/ApiExploring/AbpApiDefinitionController_Tests.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #26205      +/-   ##
==========================================
+ Coverage   49.90%   50.30%   +0.39%     
==========================================
  Files        3842     3842              
  Lines      135458   135739     +281     
  Branches    10271    10318      +47     
==========================================
+ Hits        67597    68279     +682     
+ Misses      65823    65407     -416     
- Partials     2038     2053      +15     

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