Remove MSTest.TestFramework from Core project (#3476)

## Summary

Removes the `MSTest.TestFramework` package reference from the Core
project (`src/Core/Azure.DataApiBuilder.Core.csproj`).

The Core project is not a test project and should not have any unit test
dependencies. `MSTest.TestFramework` was never used in any Core source
files — it was an unnecessary dependency that should only exist in test
projects (`Cli.Tests`, `Service.Tests`).

## Changes
- Removed `<PackageReference Include="MSTest.TestFramework" />` from
`src/Core/Azure.DataApiBuilder.Core.csproj`

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: vadeveka <52937801+vadeveka@users.noreply.github.com>

Author: Copilot

src/Core/Azure.DataApiBuilder.Core.csproj

@@ -24,7 +24,6 @@
     <PackageReference Include="Microsoft.OData.Core" />
     <PackageReference Include="Microsoft.OData.Edm" />
     <PackageReference Include="Microsoft.OpenApi" />
-    <PackageReference Include="MSTest.TestFramework" />
     <PackageReference Include="MySqlConnector" />
     <PackageReference Include="Newtonsoft.Json" />
     <PackageReference Include="Newtonsoft.Json.Schema" />