Add missing exception documentation to public API surface (#1103)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Client/StdioClientTransportOptions.cs

@@ -8,6 +8,7 @@ public sealed class StdioClientTransportOptions
     /// <summary>
     /// Gets or sets the command to execute to start the server process.
     /// </summary>
+    /// <exception cref="ArgumentException">The value is <see langword="null"/>, empty, or composed entirely of whitespace.</exception>
     public required string Command
     {
         get;