test: importorskip discord.py in the slash-gate isolation test

CI's plugin-test slice runs without the discord optional extra; the raw
import failed with ModuleNotFoundError while every other test in the
file uses injected mock modules.

tests/plugins/platforms/test_discord_gate_isolation.py

@@ -190,7 +190,9 @@ class TestSlashGateIsolation:
     def test_evaluate_slash_channel_gate_per_adapter(self, monkeypatch):
         import types
 
-        import discord as discord_lib
+        discord_lib = pytest.importorskip(
+            "discord", reason="discord.py optional dep not installed"
+        )
 
         a = _adapter()
         b = _adapter()