Fix inheritable system model replacements

[live1206]

Fixes #10787

Summary

• add first-class support for inheritable SystemObjectModelProvider replacements
• dedupe derived model properties by serialized wire name when a system base owns the property
• preserve system provider identity through visitor Update(...) calls
• register system providers by their wrapped framework type and skip model factory methods for them

Validation

• dotnet test packages\http-client-csharp\generator\Microsoft.TypeSpec.Generator\test\Microsoft.TypeSpec.Generator.Tests.csproj --filter "FullyQualifiedName~SystemObjectModelProviderTests|FullyQualifiedName~ModelFactoryProviderTests.SkipSystemObjectModelProvider" --no-restore
• npm ci && npm run build in packages\http-client-csharp
• npm run test:generator in packages\http-client-csharp
• npm test in packages\http-client-csharp

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10790

commit: 3449fb3

[github-actions]

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