refactor(V2): IBKR client read methods — defensive conid + sec_type DRY

Code review fixes (commit 611a269):
- resolve_conid validates conid key presence (was raw KeyError on malformed)
- _SEC_TYPE_MAP module constant — reused in get_ticker + get_bars
  (also fixes get_bars previously missing "forex": "CASH")
- New tests: empty response + malformed response error paths

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/exchanges/ibkr/client.py

@@ -25,6 +25,14 @@ class IBKRError(Exception):
 
 _TICKLE_INTERVAL_S = 240.0  # tickle if last call > 4min ago
 
+# Mapping asset_class (cerbero/MCP convention) → IBKR secType.
+_SEC_TYPE_MAP: dict[str, str] = {
+    "stocks": "STK",
+    "options": "OPT",
+    "futures": "FUT",
+    "forex": "CASH",
+}
+
 
 @dataclass
 class IBKRClient:
@@ -148,7 +156,12 @@ class IBKRClient:
         )
         if not result or not isinstance(result, list):
             raise IBKRError(f"IBKR_CONID_NOT_FOUND: {symbol}/{sec_type}")
-        conid = int(result[0]["conid"])
+        first = result[0]
+        if not isinstance(first, dict) or "conid" not in first:
+            raise IBKRError(
+                f"IBKR_CONID_NOT_FOUND: {symbol}/{sec_type} (malformed response)"
+            )
+        conid = int(first["conid"])
         self._conid_cache[key] = conid
         if len(self._conid_cache) > self._CONID_CACHE_MAX:
             self._conid_cache.popitem(last=False)
@@ -183,9 +196,7 @@ class IBKRClient:
     _SNAPSHOT_FIELDS = "31,84,86,7295,7296"  # last,bid,ask,bid_size,ask_size
 
     async def get_ticker(self, symbol: str, asset_class: str = "stocks") -> dict:
-        sec_type = {"stocks": "STK", "options": "OPT", "futures": "FUT", "forex": "CASH"}.get(
-            asset_class.lower(), "STK"
-        )
+        sec_type = _SEC_TYPE_MAP.get(asset_class.lower(), "STK")
         conid = await self.resolve_conid(symbol, sec_type)
         data = await self._request(
             "GET", "/iserver/marketdata/snapshot",
@@ -216,9 +227,7 @@ class IBKRClient:
         self, symbol: str, asset_class: str = "stocks",
         period: str = "1d", bar: str = "5min",
     ) -> dict:
-        sec_type = {"stocks": "STK", "options": "OPT", "futures": "FUT"}.get(
-            asset_class.lower(), "STK"
-        )
+        sec_type = _SEC_TYPE_MAP.get(asset_class.lower(), "STK")
         conid = await self.resolve_conid(symbol, sec_type)
         data = await self._request(
             "GET", "/iserver/marketdata/history",