fix: increase proxy timeout to 30s on Windows

Windows CI has slower process startup times, especially when multiple
test classes initialize their fixtures in parallel. Increase the proxy
startup timeout from 10s to 30s on Windows to reduce flakiness.

Author: jmoseley

dotnet/test/Harness/CapiProxy.cs

@@ -78,7 +78,9 @@ async Task<string> StartCoreAsync()
                 }
             });
 
-            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+            // Use longer timeout on Windows due to slower process startup
+            var timeoutSeconds = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? 30 : 10;
+            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(timeoutSeconds));
             cts.Token.Register(() => tcs.TrySetException(new TimeoutException("Timeout waiting for proxy")));
 
             return await tcs.Task;