MM-45272: Fix getPostThread permissions (#20565)

Summary
Fix permissions for the the getPostThread API Method.
User can view thread if user is member of the channel
User can view threads in public channels (in the user's team) that they're not a member of, only if compliance export is disabled.

Ticket Link
https://mattermost.atlassian.net/browse/MM-45272

api4/post_test.go

@@ -2191,10 +2191,26 @@ func TestGetPostThread(t *testing.T) {
 
 	client.RemoveUserFromChannel(th.BasicChannel.Id, th.BasicUser.Id)
 
-	// Channel is public, should be able to read post
+	messageExportEnabled := *th.App.Config().MessageExportSettings.EnableExport
+	// Channel is public, and compliance export is OFF, should be able to read post
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.MessageExportSettings.EnableExport = false
+	})
 	_, _, err = client.GetPostThread(th.BasicPost.Id, "", false)
 	require.NoError(t, err)
 
+	// channel is public, and compliance export is ON, should NOT be able to read post
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.MessageExportSettings.EnableExport = true
+	})
+	_, resp, err = client.GetPostThread(th.BasicPost.Id, "", false)
+	require.Error(t, err)
+	CheckForbiddenStatus(t, resp)
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.MessageExportSettings.EnableExport = messageExportEnabled
+	})
+
 	privatePost := th.CreatePostWithClient(client, th.BasicPrivateChannel)
 
 	_, _, err = client.GetPostThread(privatePost.Id, "", false)