bevy_internal: Apply #![deny(clippy::allow_attributes, clippy::allow_attributes_without_reason)] (#17306)

# Objective
- https://github.com/bevyengine/bevy/issues/17111

## Solution
Set the `clippy::allow_attributes` and
`clippy::allow_attributes_without_reason` lints to `deny`, and bring
`bevy_internal` in line with the new restrictions.

## Testing
`cargo clippy --tests --all-features --package bevy_internal` was run,
and no errors were encountered.

crates/bevy_internal/src/default_plugins.rs

@@ -82,7 +82,14 @@ plugin_group! {
 struct IgnoreAmbiguitiesPlugin;
 
 impl Plugin for IgnoreAmbiguitiesPlugin {
-    #[allow(unused_variables)] // Variables are used depending on enabled features
+    #[expect(
+        clippy::allow_attributes,
+        reason = "`unused_variables` is not always linted"
+    )]
+    #[allow(
+        unused_variables,
+        reason = "The `app` parameter is used only if a combination of crates that contain ambiguities with each other are enabled."
+    )]
     fn build(&self, app: &mut bevy_app::App) {
         // bevy_ui owns the Transform and cannot be animated
         #[cfg(all(feature = "bevy_animation", feature = "bevy_ui"))]

crates/bevy_internal/src/lib.rs

@@ -1,5 +1,10 @@
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![forbid(unsafe_code)]
+#![deny(
+    clippy::allow_attributes,
+    clippy::allow_attributes_without_reason,
+    reason = "See #17111; To be removed once all crates are in-line with these attributes"
+)]
 #![doc(
     html_logo_url = "https://bevyengine.org/assets/icon.png",
     html_favicon_url = "https://bevyengine.org/assets/icon.png"