Rename Handle::as_weak() to cast_weak() (#5321)

# Objective

Following discussion on #3536 and #3522, `Handle::as_weak()` takes a type `U`, reinterpreting the handle as of another asset type while keeping the same ID. This is mainly used today in font atlas code. This PR does two things:

- Rename the method to `cast_weak()` to make its intent more clear
- Actually change the type uuid in the handle if it's not an asset path variant.

## Migration Guide

- Rename `Handle::as_weak` uses to `Handle::cast_weak`

    The method now properly sets the associated type uuid if the handle is a direct reference (e.g. not a reference to an `AssetPath`), so adjust you code accordingly if you relied on the previous behavior.

crates/bevy_asset/src/handle.rs

@@ -160,9 +160,15 @@ impl<T: Asset> Handle<T> {
     }
 
     /// Recasts this handle as a weak handle of an Asset `U`.
-    pub fn as_weak<U: Asset>(&self) -> Handle<U> {
+    pub fn cast_weak<U: Asset>(&self) -> Handle<U> {
+        let id = if let HandleId::Id(_, id) = self.id {
+            HandleId::Id(U::TYPE_UUID, id)
+        } else {
+            self.id
+        };
+
         Handle {
-            id: self.id,
+            id,
             handle_type: HandleType::Weak,
             marker: PhantomData,
         }

crates/bevy_text/src/glyph_brush.rs

@@ -106,7 +106,7 @@ impl GlyphBrush {
             let section_data = sections_data[sg.section_index];
             if let Some(outlined_glyph) = section_data.1.font.outline_glyph(glyph) {
                 let bounds = outlined_glyph.px_bounds();
-                let handle_font_atlas: Handle<FontAtlasSet> = section_data.0.as_weak();
+                let handle_font_atlas: Handle<FontAtlasSet> = section_data.0.cast_weak();
                 let font_atlas_set = font_atlas_set_storage
                     .get_or_insert_with(handle_font_atlas, FontAtlasSet::default);
 

examples/ui/font_atlas_debug.rs

@@ -37,7 +37,7 @@ fn atlas_render_system(
     font_atlas_sets: Res<Assets<FontAtlasSet>>,
     texture_atlases: Res<Assets<TextureAtlas>>,
 ) {
-    if let Some(set) = font_atlas_sets.get(&state.handle.as_weak::<FontAtlasSet>()) {
+    if let Some(set) = font_atlas_sets.get(&state.handle.cast_weak::<FontAtlasSet>()) {
         if let Some((_size, font_atlas)) = set.iter().next() {
             let x_offset = state.atlas_count as f32;
             if state.atlas_count == font_atlas.len() as u32 {