Pin C# CI to .NET 8 SDK

Recent ubuntu-latest image updates changed toolchain selection in
the C# workflow. Even with setup-dotnet requesting 8.0.x, dotnet
test can run under SDK 10 and fail with MSB3030 when apphost is
missing.

Add a csharp/global.json to pin SDK resolution to .NET 8 and keep
test build behavior stable across runner image changes. Also print
dotnet version and installed SDKs in CI logs so future drift is
immediately visible.

Author: chavic

.github/workflows/csharp.yml

@@ -31,6 +31,11 @@ jobs:
         with:
           dotnet-version: "8.0.x"
 
+      - name: Verify .NET SDK
+        run: |
+          dotnet --version
+          dotnet --list-sdks
+
       - name: Generate bindings and binaries (unix)
         if: matrix.os != 'windows-latest'
         run: bash ./scripts/generate_bindings.sh

payjoin-ffi/csharp/global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "8.0.100",
+    "rollForward": "latestFeature"
+  }
+}