Use platform-aware comparer in GetDefaultEnvironmentVariables()

OrdinalIgnoreCase on Windows to match ProcessStartInfo.Environment behavior,
Ordinal on Unix where env var names are case-sensitive.

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

Author: halter73

src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs

@@ -66,7 +66,10 @@ public sealed class StdioClientTransportOptions
             ? s_defaultWindowsVars
             : s_defaultUnixVars;
 
-        var result = new Dictionary<string, string?>(StringComparer.Ordinal);
+        var result = new Dictionary<string, string?>(
+            RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
+                ? StringComparer.OrdinalIgnoreCase
+                : StringComparer.Ordinal);
         foreach (var name in names)
         {
             var value = Environment.GetEnvironmentVariable(name);