Fix C# provider-based public reference map

[ArcturusZhang]

Fixes the C# generator post-processing path to internalize generated TypeProviders before writing files. The new provider-level reference map starts from root TypeProviders, walks each canonical view's public API surface, and updates unreached public providers to internal via TypeProvider.Update instead of relying on Roslyn syntax edits.

Also removes the obsolete removeOrInternalize option; the default behavior is now internalize.

Validation:

• dotnet test generator/Microsoft.TypeSpec.Generator/test/Microsoft.TypeSpec.Generator.Tests.csproj --filter InternalizeUsesProviderPublicApiReferences --no-restore --nologo
• dotnet test generator/Microsoft.TypeSpec.Generator/test/Microsoft.TypeSpec.Generator.Tests.csproj --filter "FullyQualifiedNamePostProcessorTests|FullyQualifiedNameConfigurationTests" --no-restore --nologo
• npx vitest run -c ./emitter/vitest.config.ts emitter/test/Unit/options.test.ts
• npm run build
• npm run format -- --check
• npm run cop

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commit: 763515c

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