i've create a custom module in Magento and i want to show notice message in admin when the observer fire so my module code like below :

etc/Config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <MI_ProductAutoSynchronization>
            <version>0.0.1</version>
        </MI_ProductAutoSynchronization>
    </modules>
    <global>
        <blocks>
            <MI_ProductAutoSynchronization>
                <class>MI_ProductAutoSynchronization_Block</class>
            </MI_ProductAutoSynchronization>
        </blocks>
        <helpers>
            <MI_ProductAutoSynchronization>
                <class>MI_ProductAutoSynchronization_Helper</class>
            </MI_ProductAutoSynchronization>
        </helpers>
        <models>
             <MI_ProductAutoSynchronization>
                <class>MI_ProductAutoSynchronization_Model</class>
             </MI_ProductAutoSynchronization>
        </models>
        <events>
            <mi_productautosynchronization_notifications_before>
                <observers>
                    <mi_productautosynchronization_observer>
                        <type>singleton</type>
                        <class>MI_ProductAutoSynchronization_Model_Observer</class>
                        <method>checkMessages</method>
                    </mi_productautosynchronization_observer>
                </observers>
            </mi_productautosynchronization_notifications_before>
        </events>
    </global>
    <default>
        <MI_ProductAutoSynchronization>
          <cron_time_update>0 0 * * *</cron_time_update>
        </MI_ProductAutoSynchronization>
    </default>
    <crontab>
        <jobs>
            <MI_ProductAutoSynchronization>
                <schedule>
                  <config_path>productautosynchronization_options/section_one/cron_time_update</config_path>
                </schedule>
                <run>
                  <model>MI_ProductAutoSynchronization/observer::test</model>
                </run>
            </MI_ProductAutoSynchronization>
        </jobs>
    </crontab>
    <adminhtml>
        <acl>
            <resources>
                <admin>
                    <children>
                        <system>
                            <children>
                                <config>
                                    <children>
                                        <productautosynchronization_options>
                                            <title>Custom Configuration Section</title>
                                        </productautosynchronization_options>
                                    </children>
                                </config>
                            </children>
                        </system>
                    </children>
                </admin>
            </resources>
        </acl>
        <layout>
            <updates>
                <MI_ProductAutoSynchronization>
                    <file>mi_productautosynchronization/mi_productautosynchronization.xml</file>
                </MI_ProductAutoSynchronization>
            </updates>
        </layout>
    </adminhtml>
</config>

etc/System.xml

<?xml version="1.0"?>
<config>
    <tabs>
        <customconfig translate="label" module="MI_ProductAutoSynchronization">
            <label>Product Auto Synch Tab</label>
            <sort_order>1000002</sort_order>
        </customconfig>
    </tabs>

    <sections>
        <productautosynchronization_options translate="label" module="MI_ProductAutoSynchronization">
            <label>Configuration Settings</label>
            <tab>customconfig</tab>
            <frontend_type>text</frontend_type>
            <sort_order>1000002</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>
            <groups>
                <section_one translate="label">
                    <label>Product Time Update</label>
                    <frontend_type>text</frontend_type>
                    <sort_order>1</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>1</show_in_website>
                    <show_in_store>1</show_in_store>
                    <fields>
                        <cron_time_update translate="label">
                          <label>Products Time Update</label>
                          <frontend_type>select</frontend_type>
                          <source_model>MI_ProductAutoSynchronization/options</source_model>
                          <sort_order>1</sort_order>
                          <show_in_default>1</show_in_default>
                          <show_in_website>1</show_in_website>
                          <show_in_store>1</show_in_store>
                          <comment>Select the time to update stock automatically.</comment>
                        </cron_time_update>
                    </fields>
                </section_one>
            </groups>
        </productautosynchronization_options>
    </sections>
</config>

Block/Adminhtml/Notification.php

<?php
class MI_ProductAutoSynchronization_Block_Adminhtml_Notifications extends Mage_Adminhtml_Block_Template
{
    public function _toHtml($className = "notification-global")
    {
        // Let other extensions add messages
        Mage::dispatchEvent('mi_productautosynchronization_notifications_before');
        // Get the global notification object
        $messages = Mage::getSingleton('mi_productautosynchronization/notification')->getMessages();
        $html = null;
        foreach ($messages as $message) {
            $html .= "<div class='$className'>" . $message . "</div>";
        }
        return $html;
    }
}

Model/ Notification.php

<?php
class MI_ProductAutoSynchronization_Model_Notification extends Varien_object
{
    protected $messages = [ ];

    public function getMessages()
    {
        return $this->messages;
    }

    public function setMessages($messages)
    {
        $this->messages = $messages;
        return $this;
    }

    public function addMessage($message)
    {
        $this->messages[] = $message;
        return $this;
    }
}

Model/Observer.php

<?php
class MI_ProductAutoSynchronization_Model_Observer {

    public function test() {
      Mage::log("TEST success", null, "dev.log");
    }

    public function checkMessages($observer)
    
      Mage::log("notification success", null, "dev.log");
      $notifications = Mage::getSingleton('mi_productautosynchronization/notification');
      $notifications->addMessage("I was sent by mi_productautosynchronization");
      return $observer;
    }
}

So My Question : how i can show notice message like screen below when test function in observer fire ?