Fix IndexOutOfBoundsException in AlternativeCommandsHandler.getAlternative (#6609)

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### Information

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This PR fixes #6608. 

### Details

**Proposed fix:**    
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`getAlternative` prunes collected `WeakReference`s through its own
iterator, which can remove
every entry in a bucket. The following `commands.get(0)` then throws
`IndexOutOfBoundsException`.

This stores the first live alias during the iteration and returns that
instead, or `null` when
nothing live remains, which is what the caller already tests for
(`getAlternative(label) == null` in
`EssentialsPlayerListener.CommandSendFilter`).

Confirmed with an instrumented build that logged the list sizes around
the prune loop:

```
[6608] label=w thread=Paper Async Command Builder Thread Pool - 0
       entry=2 removedByUs=2 after=0 expected=0
```

Both references in the `w` bucket had been collected and the loop
removed both.

Note: this does not address the possible concurrent modification also
raised in #6608
(`altCommands` is a plain `HashMap` of `ArrayList`, modified from the
async command-send path).
That needs a separate synchronisation decision.


**Environments tested:**    

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OS: Debian 12

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Java version:  25.0.4 2026-07-21 LTS

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- [x] Most recent Paper version (26.2, git-Paper-112)
- [ ] CraftBukkit/Spigot/Paper 1.12.2
- [ ] CraftBukkit 1.8.8


**Demonstration:**    
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Before, on every boot with a player join (5 consecutive boots,
EssentialsX 2.22.0 and
2.22.1-dev+17-382de4f, same server and plugin set):

```
[ERROR]: Could not pass event AsyncPlayerSendCommandsEvent to Essentials
java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
    at java.util.ArrayList.get(ArrayList.java:428)
    at com.earth2me.essentials.AlternativeCommandsHandler.getAlternative(AlternativeCommandsHandler.java:133)
    at com.earth2me.essentials.EssentialsPlayerListener$CommandSendFilter.isEssentialsCommand(EssentialsPlayerListener.java:1279)
    at net.ess3.provider.providers.PaperCommandSendListenerProvider.onAsyncCommandSend(PaperCommandSendListenerProvider.java:28)
...
```

Cause isolated with a temporary instrumented build logging list sizes
around the prune loop:

```
[6608] label=w thread=Paper Async Command Builder Thread Pool - 0
       entry=2 removedByUs=2 after=0 expected=0
```

Both `WeakReference` in the `w` bucket had been collected and the loop
removed both.

After, with this fix (2.22.1-dev+18), same server, player join:

```
IndexOutOfBoundsException:                0
AlternativeCommandsHandler references:    0
Could not pass event AsyncPlayerSendCommandsEvent: 0
```

Author: minoneer

Essentials/src/main/java/com/earth2me/essentials/AlternativeCommandsHandler.java

@@ -116,6 +116,8 @@ public Command getAlternative(final String label) {
         }
 
         // return the first command that is not an alias
+        Command firstAlias = null;
+
         final Iterator<WeakReference<Command>> iterator = commands.iterator();
         while (iterator.hasNext()) {
             final Command cmd = iterator.next().get();
@@ -127,10 +129,14 @@ public Command getAlternative(final String label) {
             if (cmd.getName().equalsIgnoreCase(label)) {
                 return cmd;
             }
+
+            if (firstAlias == null) {
+                firstAlias = cmd;
+            }
         }
 
-        // return the first alias
-        return commands.get(0).get();
+        // return the first alias, or null if every reference in this bucket has been collected
+        return firstAlias;
     }
 
     public void executed(final String label, final Command pc) {