[DB MileageRate] App: Support date-aware mileage rate recalculation when editing distance request date

When editing an existing workspace distance request date, recompute the date-aware customUnitRateID and call UpdateMoneyRequestDistanceRate with created when the rate changes.

https://docs.google.com/document/d/1ByolNK4XOhNksdl_KDq7TcwJgeXgxDzW384H7SIjMU0/edit?pli=1&tab=t.0#bookmark=id.bcwwrjjvip2o

<details><summary>Issue Owner</summary>Current Issue Owner: @thelullabyy</details>