Implement StreamableHTTP transport

Co-authored-by: Topher Bullock <topher.bullock@shopify.com>

test/mcp/transport_test.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "mcp/server"
+require "mcp/transport"
+
+module MCP
+  class TransportTest < ActiveSupport::TestCase
+    class TestTransport < Transport
+      def handle_request(request)
+        [200, {}, ["OK"]]
+      end
+
+      def send_request(method, params = nil)
+        true
+      end
+
+      def close
+        true
+      end
+    end
+
+    setup do
+      @server = Server.new(
+        name: "test_server",
+        tools: [],
+        prompts: [],
+        resources: [],
+      )
+      @transport = TestTransport.new(@server)
+      # Clear transports registry before each test
+      Transport.instance_variable_set(:@transports, nil)
+    end
+
+    teardown do
+      # Clear transports registry after each test
+      Transport.instance_variable_set(:@transports, nil)
+    end
+
+    test "registers transport class" do
+      Transport.register("test", TestTransport)
+      assert_equal TestTransport, Transport.get("test")
+    end
+
+    test "raises error for unknown transport" do
+      assert_raises(RuntimeError, "Transport unknown not found") { Transport.get("unknown") }
+    end
+
+    test "initializes with server instance" do
+      assert_equal @server, @transport.instance_variable_get(:@server)
+    end
+
+    test "handles request" do
+      response = @transport.handle_request(nil)
+      assert_equal [200, {}, ["OK"]], response
+    end
+
+    test "sends request" do
+      assert @transport.send_request("test_method", { foo: "bar" })
+    end
+
+    test "closes connection" do
+      assert @transport.close
+    end
+  end
+end