fix(VAutocomplete): populate input with selected item title on initial render

[sridhar-3009]

Fixes #22270

Summary

VAutocomplete did not show the selected item's text in the input on initial render when a value was set via v-model or :model-value. The input only became populated after the user first focused the component.

Root cause

search (the text shown in the underlying input) was initialized to '' and was only updated to reflect the selected item's title inside the watch(isFocused) handler — which only fires on user interaction.

VCombobox already handles this correctly by initializing _search eagerly from the model value and watching model for external changes.

Fix

Added a watch(model, ..., { immediate: true }) in VAutocomplete.tsx that mirrors the VCombobox pattern:

watch(model, value => {
  if (!isFocused.value && !props.multiple && !hasSelectionSlot.value) {
    search.value = value.at(-1)?.props.title ?? ''
  }
}, { immediate: true })

Guards match the same conditions used in the focus watcher's focus-in branch:

• !isFocused.value — avoids overwriting the user's live typing
• !props.multiple — multiple-select doesn't show a single title in the input
• !hasSelectionSlot.value — chips/custom selection slots render separately

[J-Sek]

Is it better than #22757?

[sridhar-3009]

They solve different aspects of the same issue:

• fix(VAutocomplete): use hidden select for native form submissions #22757 mirrors the VSelect pattern from fix(VSelect): Use hidden select for native form submissions #22330 — adds a hidden <select> for native form submission and Cypress .val() support. The visible search input still starts empty until focused.
• This PR (fix(VAutocomplete): populate input with selected item title on initial render #22954) fixes the visible display — the input shows the selected item's title on initial render (before any user interaction).

Both are complementary. If #22757 lands first, this PR's fix still addresses the visual "input is empty until focused" experience. That said, if the team prefers the hidden-select pattern as the canonical approach for VAutocomplete, happy to close this in favour of #22757.

[J-Sek]

closing in favor of #23022