kucoin: handle ws welcome, subs-ack and pong msgs

Previously the subscription response handling was a bit sloppy what with
ignoring the welcome msg; this now correctly expects the correct startup
sequence. Also this avoids warn logging on pong messages by expecting
them in the msg loop and further drops the `KucoinMsg` struct and
instead changes the msg loop to expect `dict`s and only cast to structs
on live feed msgs that we actually process/relay.

piker/brokers/kucoin.py

@@ -176,17 +176,6 @@ class KucoinL2(Struct, frozen=True):
     timestamp: float
 
 
-class KucoinMsg(Struct, frozen=True):
-    '''
-    Generic outer-wrapper for any Kucoin ws msg
-
-    '''
-    type: str
-    topic: str
-    subject: str
-    data: list[KucoinTrade | KucoinL2]
-
-
 class Currency(Struct, frozen=True):
     '''
     Currency (asset) info:
@@ -743,12 +732,14 @@ async def subscribe(
                 'id': connect_id,
                 'type': 'subscribe',
                 'topic': ep,
-                # 'topic': f'/spotMarket/level2Depth5:{bs_mktid}',
                 'privateChannel': False,
                 'response': True,
             }
         )
 
+    welcome_msg = await ws.recv_msg()
+    log.info(f'WS welcome: {welcome_msg}')
+
     for _ in topics:
         ack_msg = await ws.recv_msg()
         log.info(f'Sub ACK: {ack_msg}')
@@ -782,19 +773,16 @@ async def stream_messages(
     '''
     last_trade_ts: float = 0
 
+    dict_msg: dict[str, Any]
     async for dict_msg in ws:
-        if 'subject' not in dict_msg:
-            log.warn(f'Unhandled message: {dict_msg}')
-            continue
+        match dict_msg:
+            case {
+                'subject': 'trade.ticker',
+                'data': trade_data_dict,
+            }:
+                trade_data = KucoinTrade(**trade_data_dict)
 
-        msg = KucoinMsg(**dict_msg)
-        match msg:
-            case KucoinMsg(
-                subject='trade.ticker',
-            ):
-                trade_data = KucoinTrade(**msg.data)
-
-                # XXX: Filter for duplicate messages as ws feed will
+                # XXX: Filter out duplicate messages as ws feed will
                 # send duplicate market state
                 # https://docs.kucoin.com/#level2-5-best-ask-bid-orders
                 if trade_data.time == last_trade_ts:
@@ -816,10 +804,11 @@ async def stream_messages(
                     ],
                 }
 
-            case KucoinMsg(
-                subject='level2',
-            ):
-                l2_data = KucoinL2(**msg.data)
+            case {
+                'subject': 'level2',
+                'data': trade_data_dict,
+            }:
+                l2_data = KucoinL2(**trade_data_dict)
                 first_ask = l2_data.asks[0]
                 first_bid = l2_data.bids[0]
                 yield 'l1', {
@@ -848,8 +837,12 @@ async def stream_messages(
                     ],
                 }
 
+            case {'type': 'pong'}:
+                # resp to ping task req
+                continue
+
             case _:
-                log.warn(f'Unhandled message: {msg}')
+                log.warn(f'Unhandled message: {dict_msg}')
 
 
 @acm