Add README for netstandard2.0 test project

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Netstandard2_0.Tests/README.md

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+# ModelContextProtocol.Netstandard2_0.Tests
+
+This test project provides cross-platform testing of the `netstandard2.0` build of `ModelContextProtocol.Core`.
+
+## Purpose
+
+The main test project (`ModelContextProtocol.Tests`) targets multiple frameworks including `net472`, which only runs on Windows. This creates a gap for developers on non-Windows platforms who want to test the `netstandard2.0` build.
+
+This project solves that by:
+1. Targeting `net8.0` (stable, cross-platform)
+2. Forcing `ModelContextProtocol.Core` to use its `netstandard2.0` build via `SetTargetFramework`
+3. Including all test source files from the main test project
+
+## Key Features
+
+- **Cross-platform**: Runs on Windows, Linux, and macOS using the modern .NET runtime
+- **netstandard2.0 testing**: Verifies the netstandard2.0 build without requiring .NET Framework or mono
+- **Shared tests**: Uses the same test source files as `ModelContextProtocol.Tests` to ensure consistency
+
+## How It Works
+
+The project uses the MSBuild `SetTargetFramework` property on the project reference to force the referenced project to build for a specific target framework:
+
+```xml
+<ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj">
+  <SetTargetFramework>TargetFramework=netstandard2.0</SetTargetFramework>
+</ProjectReference>
+```
+
+This allows the test project (targeting `net8.0`) to reference and test the `netstandard2.0` build of `ModelContextProtocol.Core`.
+
+## Running Tests
+
+```bash
+# Run all tests
+dotnet test tests/ModelContextProtocol.Netstandard2_0.Tests
+
+# Run with filter
+dotnet test tests/ModelContextProtocol.Netstandard2_0.Tests --filter '(Execution!=Manual)'
+```
+
+## Reference
+
+This pattern is based on the approach used by [googleapis/dotnet-genai](https://github.com/googleapis/dotnet-genai/blob/f0d9a3eb970e91293b806dac49853b68e4ddcdca/Google.GenAI.Tests/Netstandard2_0Tests/Google.GenAI.Netstandard2_0.Tests.csproj).