feat(19.6.1-05): persist picked relation labels across FormView tab remount

- FORM_REMEMBER_LABEL inject so hosts keep picked option names
- RelationField choose() writes the label before emit
- Vitest backstops remount and tab-switch labeling

Co-authored-by: Cursor <cursoragent@cursor.com>

admin/tests/form/RelationField.test.ts

@@ -1,7 +1,9 @@
 import { afterEach, beforeEach, describe, expect, it } from 'vitest'
 import { enableAutoUnmount, flushPromises, mount, type VueWrapper } from '@vue/test-utils'
 import { setBundle } from '../../src/app/i18n'
+import type { RelationOption } from '../../src/api/types'
 import RelationField from '../../src/components/form/fields/RelationField.vue'
+import { FORM_REMEMBER_LABEL } from '../../src/components/form/formContext'
 import { formField, langFixture, optionsFixture, recordFixture } from '../fixtures/typed'
 import { API, keydown, mockApi, queryOf, requestsTo, resetState, wait, type Route } from '../helpers'
 
@@ -243,6 +245,48 @@ describe('multiple relation (D-18)', () => {
   })
 })
 
+describe('remembered relation labels across remount', () => {
+  it('shows a picked label after remount when the form remembered it', async () => {
+    mockApi(routes())
+    const previous: RelationOption[] = [{ value: 2, label: 'Bolt & Co' }]
+    const bag: RelationOption[] = []
+    const wrapper = mount(RelationField, {
+      props: {
+        field: formField('maker'),
+        modelValue: 2,
+        controlId: 'r-maker',
+        labels: previous,
+        source,
+      },
+      global: {
+        provide: {
+          [FORM_REMEMBER_LABEL as symbol]: (_field: string, option: RelationOption) => {
+            bag.push(option)
+          },
+        },
+      },
+      attachTo: document.body,
+    })
+    await openList(wrapper)
+    await wrapper.find('[role="option"][data-value="1"]').trigger('mousedown')
+    expect(bag).toEqual([expect.objectContaining({ value: 1, label: 'Acme Works' })])
+    wrapper.unmount()
+
+    const remounted = mount(RelationField, {
+      props: {
+        field: formField('maker'),
+        modelValue: 1,
+        controlId: 'r-maker',
+        labels: [...previous, ...bag],
+        source,
+      },
+      attachTo: document.body,
+    })
+    expect((combobox(remounted).element as HTMLInputElement).value).toBe('Acme Works')
+    expect((combobox(remounted).element as HTMLInputElement).value).not.toBe('1')
+  })
+})
+
 describe('locked relation options (UI-SPEC S6, D-07)', () => {
   const NOTE = 'Items marked with a lock need an additional permission to change.'
   const strings = {