[DB MileageRate] App: Add date bounds to mileage rate create flow

[rlinoz]

Add startDate/endDate to policy rate types, optimistic create data, CreatePolicyDistanceRate params, create form inputs, validation, and submit payload.

https://docs.google.com/document/d/1ByolNK4XOhNksdl_KDq7TcwJgeXgxDzW384H7SIjMU0/edit?pli=1&tab=t.0#bookmark=id.93zvt9lp75b4

Issue Owner

Current Issue Owner: @thelullabyy

[Krishna2323]

@rlinoz The draft PR is up: #90153. The BE doesn't seem to be persisting the new fields (startDate, endDate) yet — are we blocked on that, or can the FE work proceed independently? Also, does this feature need a beta gate?

Monosnap.screencast.2026-05-11.22-35-56.mp4

[c3024]

Backend PRs are in review. They will likely be merged today. They should be on staging in a day or two.

[c3024]

Backend PRs are on staging.

[rlinoz]

Hey @Krishna2323 , sorry for the delay.

I think a beta makes a lot of sense. I will push a beta PR for backend today, let's call it dateBoundMileageRate.

[Krishna2323]

@rlinoz @neil-marcellini who will be reviewing the PR? The code changes are ready, and I’ll open it for review once I’ve added the recordings.

[rlinoz]

@thelullabyy will be reviewing the App PRs

[Krishna2323]

@Expensify/design The spec shows the date validation error ("Start date must occur before end date") above the Save button. However, in this PR implementation we show inline errors under each field (Name, Amount, Start date) since the form can have multiple validation errors at once — displaying only one error above Save could be confusing when there are 3+ issues to fix.

PR current behavior:

• Inline errors under each invalid field (so the user sees all issues at once)
• "Please fix the errors in the form before continuing." alert above Save

Is this acceptable, or should we match the spec exactly and show only the specific date error above Save?

Monosnap.screencast.2026-05-15.16-58-02.mp4