Fall back to `Contract.exchange` in `has_holiday()`

Use `con.exchange` as fallback when `con.primaryExchange` is empty
in `has_holiday()` to handle contracts like futures that don't
always set a `primaryExchange`.

Deats,
- extract `con: Contract` from `con_deats.contract` for reuse.
- use `con.primaryExchange or con.exchange` to ensure a valid
  exchange code is always passed to the calendar lookup.
- add `Contract` to `TYPE_CHECKING` imports.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/brokers/ib/venues.py

@@ -43,6 +43,7 @@ from pendulum import (
 if TYPE_CHECKING:
     from ib_async import (
         TradingSession,
+        Contract,
         ContractDetails,
     )
     from exchange_calendars.exchange_calendars import (
@@ -82,7 +83,12 @@ def has_holiday(
 
     '''
     tz: str = con_deats.timeZoneId
-    exch: str = con_deats.contract.primaryExchange
+    con: Contract = con_deats.contract
+    exch: str = (
+        con.primaryExchange
+        or
+        con.exchange
+    )
 
     # XXX, ad-hoc handle any IB exchange which are non-std
     # via lookup table..