chore(ci): enable NuGet cache and bump Microsoft.NET.Test.Sdk to 18.3.0

[Copilot]

This PR applies the Repo Assist update by adding NuGet package caching to CI and updating Microsoft.NET.Test.Sdk to the latest requested version. Scope is intentionally limited to workflow/package-version changes.

• CI: NuGet caching via setup-dotnet

  • Updated .github/workflows/dotnet.yml to enable built-in NuGet cache in actions/setup-dotnet@v4.
  • Cache keying is tied to central package version definitions through Directory.Packages.props.

• Dependencies: test SDK refresh

  • Updated Microsoft.NET.Test.Sdk in Directory.Packages.props from 18.0.1 to 18.3.0.

- name: Setup .NET
  uses: actions/setup-dotnet@v4
  with:
    dotnet-version: 10.0.x
    cache: true
    cache-dependency-path: '**/Directory.Packages.props'