mcp: remove json-rpc batching for more recent protocol versions

JSON-RPC batching support is removed for protocol versions greater than or
equal to protocolVersion20250618

Fixes #21

mcp/streamable_test.go

@@ -647,6 +647,46 @@ func TestStreamableServerTransport(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "batch rejected on 2025-06-18",
+			requests: []streamableRequest{
+				initialize,
+				initialized,
+				{
+					method: "POST",
+					// Explicitly set the protocol version header
+					headers: http.Header{"MCP-Protocol-Version": {"2025-06-18"}},
+					// Two messages => batch. Expect reject.
+					messages: []jsonrpc.Message{
+						req(101, "tools/call", &CallToolParams{Name: "tool"}),
+						req(102, "tools/call", &CallToolParams{Name: "tool"}),
+					},
+					wantStatusCode:     http.StatusBadRequest,
+					wantBodyContaining: "batch",
+				},
+			},
+		},
+		{
+			name: "batch accepted on 2025-03-26",
+			requests: []streamableRequest{
+				initialize,
+				initialized,
+				{
+					method:  "POST",
+					headers: http.Header{"MCP-Protocol-Version": {"2025-03-26"}},
+					// Two messages => batch. Expect OK with two responses in order.
+					messages: []jsonrpc.Message{
+						req(201, "tools/call", &CallToolParams{Name: "tool"}),
+						req(202, "tools/call", &CallToolParams{Name: "tool"}),
+					},
+					wantStatusCode: http.StatusOK,
+					wantMessages: []jsonrpc.Message{
+						resp(201, &CallToolResult{Content: []Content{}}, nil),
+						resp(202, &CallToolResult{Content: []Content{}}, nil),
+					},
+				},
+			},
+		},
 		{
 			name: "tool notification",
 			tool: func(t *testing.T, ctx context.Context, ss *ServerSession) {