fix(qqofficial): tolerate missing optional token in adapter config (#2496)

Since b55f073e the token field is optional in qqofficial.yaml ("the
current adapter implementation does not use it either, so it can be
safely left blank"), but the adapter constructor still reads it with
config['token']. Creating a bot via QR binding (which only returns
appid/secret) or with the token field left blank raises KeyError and
the API returns 500.

Read it with config.get('token', '') instead. The value is never used
by QQOfficialClient beyond being stored, so an empty string default is
safe.

src/langbot/pkg/platform/sources/qqofficial.py

@@ -205,7 +205,7 @@ class QQOfficialAdapter(abstract_platform_adapter.AbstractMessagePlatformAdapter
         bot = QQOfficialClient(
             app_id=config['appid'],
             secret=config['secret'],
-            token=config['token'],
+            token=config.get('token', ''),
             logger=logger,
             unified_mode=enable_webhook,
         )