🐛 fix union validation error (#1001)

tests/plugins/param/param_bot.py

@@ -1,3 +1,5 @@
+from typing import Union
+
 from nonebot.adapters import Bot
 
 
@@ -5,9 +7,29 @@ async def get_bot(b: Bot) -> Bot:
     return b
 
 
-class SubBot(Bot):
+async def legacy_bot(bot):
+    return bot
+
+
+async def not_legacy_bot(bot: int):
     ...
 
 
-async def sub_bot(b: SubBot) -> SubBot:
+class FooBot(Bot):
+    ...
+
+
+async def sub_bot(b: FooBot) -> FooBot:
     return b
+
+
+class BarBot(Bot):
+    ...
+
+
+async def union_bot(b: Union[FooBot, BarBot]) -> Union[FooBot, BarBot]:
+    return b
+
+
+async def not_bot(b: Union[int, Bot]):
+    ...