Return 202 Accepted for SSE responses per MCP spec

`StreamableHTTPTransport#send_response_to_stream` returned 200 with
`{ accepted: true }` body, but the MCP spec requires 202 with empty body
when a response is sent via SSE stream. This has been a requirement since
Streamable HTTP was introduced in protocol version 2025-03-26.

- https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server
- https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -288,7 +288,7 @@ def get_session_stream(session_id)
         def send_response_to_stream(stream, response, session_id)
           message = JSON.parse(response)
           send_to_stream(stream, message)
-          [200, { "Content-Type" => "application/json" }, [{ accepted: true }.to_json]]
+          handle_accepted
         rescue IOError, Errno::EPIPE => e
           MCP.configuration.exception_reporter.call(
             e,

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -853,6 +853,47 @@ class StreamableHTTPTransportTest < ActiveSupport::TestCase
           assert_nil(body)
         end
 
+        test "send_response_to_stream returns 202 when message is sent to stream" do
+          # Create and initialize a session
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "123" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          # Connect with SSE
+          io = StringIO.new
+          get_request = create_rack_request(
+            "GET",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(get_request)
+          response[2].call(io) if response[2].is_a?(Proc)
+
+          # Give the stream time to set up
+          sleep(0.1)
+
+          # Make a regular request that will be routed through send_response_to_stream
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => session_id,
+            },
+            { jsonrpc: "2.0", method: "ping", id: "456" }.to_json,
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 202, response[0]
+          assert_empty response[1]
+          assert_empty response[2]
+        end
+
         test "handle post request with a standard error" do
           request = create_rack_request(
             "POST",