Fix PluginBrigadierCommand suggestions delegation

src/main/java/io/papermc/paperweight/testplugin/PluginBrigadierCommand.java

@@ -1,9 +1,15 @@
 package io.papermc.paperweight.testplugin;
 
+import com.mojang.brigadier.CommandDispatcher;
+import com.mojang.brigadier.ParseResults;
+import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.builder.LiteralArgumentBuilder;
+import java.util.List;
 import java.util.function.Consumer;
+import com.mojang.brigadier.suggestion.Suggestion;
 import net.minecraft.commands.CommandSourceStack;
 import org.bukkit.Bukkit;
+import org.bukkit.Location;
 import org.bukkit.command.Command;
 import org.bukkit.command.CommandSender;
 import org.bukkit.command.PluginIdentifiableCommand;
@@ -11,6 +17,7 @@ import org.bukkit.craftbukkit.v1_19_R1.CraftServer;
 import org.bukkit.craftbukkit.v1_19_R1.command.VanillaCommandWrapper;
 import org.bukkit.plugin.Plugin;
 import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 import org.checkerframework.framework.qual.DefaultQualifier;
 
 @DefaultQualifier(NonNull.class)
@@ -40,6 +47,17 @@ final class PluginBrigadierCommand extends Command implements PluginIdentifiable
     return true;
   }
 
+  @Override
+  public List<String> tabComplete(final CommandSender sender, final String alias, final String[] args, final @Nullable Location location) {
+    final String joined = String.join(" ", args);
+    final String argsString = joined.isBlank() ? "" : joined;
+    final CommandDispatcher<CommandSourceStack> dispatcher = ((CraftServer) Bukkit.getServer()).getServer().getCommands().getDispatcher();
+    final ParseResults<CommandSourceStack> results = dispatcher.parse(new StringReader(alias + " " + argsString), VanillaCommandWrapper.getListener(sender));
+    return dispatcher.getCompletionSuggestions(results)
+      .thenApply(result -> result.getList().stream().map(Suggestion::getText).toList())
+      .join();
+  }
+
   @Override
   public Plugin getPlugin() {
     return this.plugin;