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