GH-9633: MailReceiver: also set flags into IntegrationMimeMessage

Fixes: #9633
Issue link: #9633

The message flags of the received messages are outdated and don't contain the changes of `AbstractMailReceiver#setMessageFlags`,
because the flags are only applied to the original message and not the copy (`AbstractMailReceiver.IntegrationMimeMessage`).

* Call `setMessageFlagsAndMaybeDeleteMessages()` for `IntegrationMimeMessage` copies as well

**Auto-cherry-pick to `6.3.x` & `6.2.x`**

spring-integration-mail/src/main/java/org/springframework/integration/mail/AbstractMailReceiver.java

@@ -529,6 +529,9 @@ private void postProcessFilteredMessages(Message[] filteredMessages) throws Mess
 		}
 
 		setMessageFlagsAndMaybeDeleteMessages(messages);
+		if (filteredMessages.length > 0 && filteredMessages[0] instanceof IntegrationMimeMessage) {
+			setMessageFlagsAndMaybeDeleteMessages(filteredMessages);
+		}
 	}
 
 	private void setMessageFlagsAndMaybeDeleteMessages(Message[] messages) throws MessagingException {

spring-integration-mail/src/test/java/org/springframework/integration/mail/ImapMailReceiverTests.java

@@ -449,7 +449,7 @@ public void receiveMarkAsReadAndDelete() throws Exception {
 		assertThat(msg1.getFlags().contains(Flag.SEEN)).isTrue();
 		assertThat(msg2.getFlags().contains(Flag.SEEN)).isTrue();
 
-		verify(receiver, times(1)).deleteMessages(Mockito.any());
+		verify(receiver, times(2)).deleteMessages(Mockito.any());
 	}
 
 	@Test