build: pin SDK to .NET 9 via global.json

Prevents .NET 10 SDK from being picked up
automatically, which causes WinRT.Runtime version
conflicts (MSB3277) when building net9.0 targets.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sundaramramaswamy

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "9.0.100",
+    "rollForward": "latestFeature"
+  }
+}