feat(VAlert): add duration prop to auto-dismiss alerts

[eldardada]

Description

Adds a duration prop to VAlert that automatically dismisses the alert after a given time in milliseconds. Currently developers have to wire up their own setTimeout to hide an alert after a delay; this brings the behavior in line with VSnackbar's timeout.

• duration accepts a number (or numeric string) of milliseconds.
• 0 (default) keeps the current behavior and never auto-dismisses, so the change is fully backwards compatible.
• The timer restarts when the alert becomes visible again or when duration changes, and is cleared on manual close and on unmount — mirroring VSnackbar's timeout handling.

A unit test covers both auto-dismiss and the duration="0" (persistent) case, and the API description has been added.

resolves #20409

Markup:

<template>
  <v-btn @click="visible = true">Show alert</v-btn>
  <v-alert
    v-model="visible"
    :duration="3000"
    closable
    type="success"
    text="I will dismiss myself after 3 seconds"
  />
</template>

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

  const visible = shallowRef(true)
</script>

[eldardada]

On the prop name: I used duration as asked in #20409, though VSnackbar already calls the same thing timeout. The "off" value differs too: 0 (the default) means "never dismiss" here so existing alerts keep working, whereas VSnackbar defaults to 5000 and uses -1 for never. Happy to rename to timeout and match that convention if you'd rather the two line up.

[eldardada]

On pausing: I copied the hover/focus handling from VSnackbar (pointerenter/leave + focusin/out), so an alert with a close button or links in a slot doesn't vanish while someone's reading it or reaching to click. Covers WCAG 2.2.1 as a side effect.

[eldardada]

On reuse: I kept the timer inline rather than extracting a shared helper. VSnackbar's useCountdown is local and not exported, so sharing it would mean touching VSnackbar and pushing this past a good first issue. Can split that out separately if you want the two unified.

[eldardada]

On edge values: Infinity and values past the 32-bit setTimeout limit dismiss right away, same as VSnackbar today. Left it for parity, can add a guard if you'd prefer.