[csharp] Fix/override central package versions for csharp test projects

[wturk]

The generated test .csproj was missing <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally> in its <PropertyGroup>, unlike the main client .csproj which already includes it. This caused build failures in repositories that enforce Central Package Management (CPM).
This PR updates the test project template to match the client project template by adding the missing property.

Fixes #23090

@mandrean @shibayan @Blackclaws @iBicha

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Summary by cubic

Adds ManagePackageVersionsCentrally=false to generated C# test project templates to override Central Package Management and prevent build failures in CPM-enforced repos. Regenerates C# samples to include this property. Fixes #23090.

• Bug Fixes
  • Add ManagePackageVersionsCentrally=false to netcore_testproject.mustache.
  • Align test .csproj with client .csproj so tests build under CPM.

^{Written for commit 889d8fb. Summary will update on new commits.}

[cubic-dev-ai]

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