Automatically fall back from Streamable HTTP to SSE on the client by default (#456)

src/ModelContextProtocol/Client/HttpTransportMode.cs

@@ -0,0 +1,23 @@
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Specifies the transport mode for HTTP client connections.
+/// </summary>
+public enum HttpTransportMode
+{
+    /// <summary>
+    /// Automatically detect the appropriate transport by trying Streamable HTTP first, then falling back to SSE if that fails.
+    /// This is the recommended mode for maximum compatibility.
+    /// </summary>
+    AutoDetect,
+
+    /// <summary>
+    /// Use only the Streamable HTTP transport.
+    /// </summary>
+    StreamableHttp,
+
+    /// <summary>
+    /// Use only the HTTP with SSE transport.
+    /// </summary>
+    Sse
+}