fix(mangler): use retain instead of truncate to remove empty frequency slots (#18225)

#18183 introduced a bug when removing empty frequency slots. The code used `truncate` after sorting:

```rust
frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
if let Some(idx) = frequencies.iter().position(|x| x.symbol_ids.is_empty()) {
    frequencies.truncate(idx);
}
```

The problem is that `sort_unstable_by_key` doesn't guarantee order for items with equal keys. When multiple slots have `frequency: 0`, some have symbols (that need renaming but have no references), and some are empty. Since the sort order is unstable, an empty slot could appear **before** a non-empty slot, and `truncate` would incorrectly remove valid slots.

This fix uses `retain` instead, and moves it before the sort for better efficiency (sorting fewer elements):

```rust
frequencies.retain(|x| !x.symbol_ids.is_empty());
frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
```

Note for reviewer: I've locally tested in the `monitor-oxc`; there is no naming collision after this.

crates/oxc_mangler/src/lib.rs

@@ -570,10 +570,10 @@ impl<'t> Mangler<'t> {
             frequencies[index].frequency += scoping.get_resolved_reference_ids(symbol_id).len();
             frequencies[index].symbol_ids.push(symbol_id);
         }
+
+        // Remove slots that have no symbols to rename before sorting.
+        frequencies.retain(|x| !x.symbol_ids.is_empty());
         frequencies.sort_unstable_by_key(|x| std::cmp::Reverse(x.frequency));
-        if let Some(idx) = frequencies.iter().position(|x| x.symbol_ids.is_empty()) {
-            frequencies.truncate(idx);
-        }
         frequencies
     }
 

crates/oxc_minifier/tests/mangler/snapshots/mangler.snap

@@ -18,7 +18,7 @@ function foo(e) {
 
 var x; function foo(a) { ({ x } = y) }
 var x;
-function foo(a) {
+function foo(e) {
 	({x} = y);
 }