fix: stable ids for SelectionGroup date fields

HDS SelectionGroup unwraps a single wrapper child and renders the
grandchildren in a list keyed by `child.props.id`. The two date-field
Controllers had no id, producing undefined keys and a "unique key prop"
warning under React 19's stricter dev validation (the @axe-core/react
createElement wrapper surfaced it in the stack). Give each Controller a
stable id so HDS has a real key.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Riippi

src/components/exception-opening-hours-validity/ExceptionOpeningHoursValidity.tsx

@@ -32,9 +32,12 @@ const ExceptionOpeningHoursValidity = (): JSX.Element => {
           label={t('OpeningHours.ExceptionOpeningHoursValidity')}
           required>
           <div className="exception-opening-hours-validity__dates">
+            {/* HDS SelectionGroup keys its children by props.id, so give
+                each field a stable id to avoid a missing-key warning. */}
             <Controller
               defaultValue={startDate ?? ''}
               name="startDate"
+              {...{ id: 'exception-start-date-field' }}
               render={({
                 field: { name, onBlur, onChange, ref, value },
                 fieldState,
@@ -75,6 +78,7 @@ const ExceptionOpeningHoursValidity = (): JSX.Element => {
             <Controller
               defaultValue={endDate ?? ''}
               name="endDate"
+              {...{ id: 'exception-end-date-field' }}
               render={({
                 field: { name, onBlur, onChange, ref, value },
                 fieldState,