Fix is_plugin_added::<Self>() being true during build (#13817)

# Objective

Fixes #13815 

## Solution

Move insertion of the plugin name to after build is called.

## Testing

I added a regression test

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: François Mockers <mockersf@gmail.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>

Author: 3 people

crates/bevy_app/src/app.rs

@@ -439,12 +439,7 @@ impl App {
         plugin: Box<dyn Plugin>,
     ) -> Result<&mut Self, AppError> {
         debug!("added plugin: {}", plugin.name());
-        if plugin.is_unique()
-            && !self
-                .main_mut()
-                .plugin_names
-                .insert(plugin.name().to_string())
-        {
+        if plugin.is_unique() && self.main_mut().plugin_names.contains(plugin.name()) {
             Err(AppError::DuplicatePlugin {
                 plugin_name: plugin.name().to_string(),
             })?;
@@ -459,6 +454,9 @@ impl App {
 
         self.main_mut().plugin_build_depth += 1;
         let result = catch_unwind(AssertUnwindSafe(|| plugin.build(self)));
+        self.main_mut()
+            .plugin_names
+            .insert(plugin.name().to_string());
         self.main_mut().plugin_build_depth -= 1;
 
         if let Err(payload) = result {
@@ -1275,6 +1273,21 @@ mod tests {
             .init_resource::<TestResource>();
     }
 
+    #[test]
+    /// Plugin should not be considered inserted while it's being built
+    ///
+    /// bug: <https://github.com/bevyengine/bevy/issues/13815>
+    fn plugin_should_not_be_added_during_build_time() {
+        pub struct Foo;
+
+        impl Plugin for Foo {
+            fn build(&self, app: &mut App) {
+                assert!(!app.is_plugin_added::<Self>());
+            }
+        }
+
+        App::new().add_plugins(Foo);
+    }
     #[test]
     fn events_should_be_updated_once_per_update() {
         #[derive(Event, Clone)]