fix panic looking for extension in multibyte unicode paths (#19933)

# Objective

```
2025-07-03T11:48:34.039501Z ERROR panic: thread 'IO Task Pool (6)' panicked at 'byte index 9 is not a char boundary; it is inside '个' (bytes 7..10) of `展示_个人收款码.png`': [...]\crates\bevy_asset\src\path.rs:475
```

## Solution

char_indices

crates/bevy_asset/src/path.rs

@@ -480,7 +480,7 @@ impl<'a> AssetPath<'a> {
     }
 
     pub(crate) fn iter_secondary_extensions(full_extension: &str) -> impl Iterator<Item = &str> {
-        full_extension.chars().enumerate().filter_map(|(i, c)| {
+        full_extension.char_indices().filter_map(|(i, c)| {
             if c == '.' {
                 Some(&full_extension[i + 1..])
             } else {