Cleanup post embed for WebSocket payload (#27763)

* Cleanup post embed for WebSocket payload

* Removing post embeds in create post API

* Fixed a nil check:

* Removed a now-irrelavent test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/post.go

@@ -508,6 +508,10 @@ func (o *Post) SanitizeProps() {
 func (o *Post) SanitizeInput() {
 	o.DeleteAt = 0
 	o.RemoteId = NewPointer("")
+
+	if o.Metadata != nil {
+		o.Metadata.Embeds = nil
+	}
 }
 
 func (o *Post) ContainsIntegrationsReservedProps() []string {