[EuiDatePickerRange] Expose the form control layout's `clear` prop

**Requestor**

Myself (@ChrisJamesC).

**Problem Statement**

`EuiDatePickerRange` has no way to render a clear button. Every other control
built on `EuiFormControlLayout` can expose one via `clear`, and the single
`EuiDatePicker` has `onClear` — but the range variant forwards neither, so
there is no way to let users reset a selected range from within the control.

**Proposed Solution**

Forward `EuiFormControlLayoutDelimited`'s existing `clear` prop through
`EuiDatePickerRange`, by adding it to the `Pick`
list the component already uses for `isLoading`, `isInvalid`, `prepend`,
`append`, and `delimiter`. No new abstraction, no new UI — it reuses
`EuiFormControlLayoutClearButton` as-is.

One open design question worth deciding here: `EuiDatePicker` exposes
`onClear` (a handler) whereas this would expose `clear` (`{ onClick }`).
Forwarding `clear` is consistent with how `EuiDatePickerRange` forwards its
other layout props; exposing `onClear` would be consistent with the sibling
single-date component. Happy to implement either — I'd defer to the team.

I previously opened #8428 with the prop-forwarding change. It was closed
after going inactive, and I understand from #9960 that community
contributions are accepted for issues labelled `help wanted` — hence this
issue rather than reopening that PR directly.

**Use Case**

Any form with an optional date range: filters, report parameters, scheduling.
Users can select a range but cannot unset it without the consuming app
building its own reset affordance outside the control, which then sits
inconsistently next to other clearable inputs on the same form.

**Value / Impact**

Value to library consumers, mainly as consistency: it closes a gap where the
date range is the odd one out among form controls. The implementation cost is
small — forwarding an existing prop — and the risk is low, since the prop is
optional and undefined by default, so existing usage renders identically.

**Urgency**

Low. Not blocking us; we render our own reset button today.

**Do alternatives or workarounds exist?**

Yes, two, both unsatisfying:
- Render a separate reset button outside the control, which looks inconsistent
next to other inputs that clear inline.
- Use `isCustom` and assemble the layout manually, which means reimplementing
what `EuiDatePickerRange` already does.

**Related code or customizations**

- Previous PR: #8428
- Closest prior issue: #5082 (a different concern — clear buttons on the inner
inputs overlapping the value at narrow widths)

**Additional context (Optional)**

If this is something the team wants and gets labelled `help wanted`, I'm happy
to open a follow-up PR addressing all the suggestions from #8428 — @weronikaolejniczak
fairly asked there for a changelog entry, a unit test for clearing, and confirmation
that the prop is available as a Storybook control. I have those done already on
a branch rebased onto current `main`, so it's ready whenever it's wanted.

Start at EuiDatePickerRange's Pick list and compare the existing clear behavior of EuiDatePicker, EuiFormControlLayoutDelimited, and EuiFormControlLayoutClearButton; review previous PR #8428 for context. Done means the range component forwards clear, with the related unit test, changelog entry, and Storybook control confirmed.

Written by the indexing model from the issue text.