fix(VCommandPalette): pass keyboard and click wiring to custom slots

[MatthewAry]

Description

fixes #23259

Depends on #23258 and is based on its branch, so only the slot changes show here.

A custom input from the input slot had no keyboard navigation: the keydown handler (and now the combobox attributes) were only attached to the built-in VTextField. Clicking an item rendered through the item slot didn't run the command either.

Both slots now pass props to bind, like VSelect's item slot and VOverlay's activator slot:

• input: { props } with role, aria-expanded, aria-controls, aria-autocomplete, aria-activedescendant and onKeydown. The built-in VTextField spreads the same object. A custom input is also focused when the palette opens.
• item: { item, index, props }. props holds the same list item fields the default rendering uses (value, title, subtitle, icons, avatars) plus index and an onClick that runs the command, so other item fields like hotkey don't leak onto the DOM. onClick replaces the item's own handler instead of merging with it, because running the command already calls item.onClick(event, value).

props doesn't carry the search model: a VTextField needs modelValue, a native input needs value/onInput, and the built-in field already has its own v-model. The docs and slot descriptions say to connect a custom input to v-model:search instead.

Behavior change: item in the item, item.prepend, item.title and item.append slots is now the original item, matching the declared slot type. Before, it was the internal list props, where value was the filtered index.

Adds slot-input and slot-item docs examples.

Markup:

<template>
  <v-app theme="dark">
    <v-container max-width="600">
      <div class="d-flex ga-4 mb-4">
        <v-btn text="Custom input" @click="inputOpen = true" />
        <v-btn text="Custom item" @click="itemOpen = true" />
      </div>

      <code>click:item → {{ last || 'none' }}</code>

      <v-command-palette
        v-model="inputOpen"
        v-model:search="search"
        :items="items"
        @click:item="last = $event.title"
      >
        <template #input="{ props }">
          <input v-model="search" v-bind="props" class="pa-4 w-100" placeholder="Search">
        </template>
      </v-command-palette>

      <v-command-palette
        v-model="itemOpen"
        :items="items"
        @click:item="last = $event.title"
      >
        <template #item="{ props }">
          <v-list-item v-bind="props" />
        </template>
      </v-command-palette>
    </v-container>
  </v-app>
</template>

<script setup>
  import { shallowRef } from 'vue'

  const inputOpen = shallowRef(false)
  const itemOpen = shallowRef(false)
  const search = shallowRef('')
  const last = shallowRef('')

  const items = [
    { title: 'New file', value: 'new-file' },
    { title: 'Open file', value: 'open-file' },
    { title: 'Sign out', value: 'sign-out' },
  ]
</script>

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