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

[eldardada]

Description

Adds a timeout prop to VAlert that automatically dismisses the alert after a given time in milliseconds, matching VSnackbar's timeout. Currently developers have to wire up their own setTimeout to hide an alert after a delay.

• timeout accepts a number (or numeric string) of milliseconds.
• -1 (the default) keeps the alert visible indefinitely, so existing alerts are unaffected. 0 dismisses immediately and > 0 dismisses after the given delay, matching VSnackbar's semantics.
• The timer pauses while the alert is hovered or focused, so it doesn't vanish while someone is reading it or reaching for the close button / a link in a slot. It restarts when the alert becomes visible again or when timeout changes, and is cleared on manual close and on unmount.

The auto-dismiss timeout and pause-on-hover/focus handling is shared with VSnackbar through a new useAutoDismiss composable, so the two components stay consistent and the hover/focus pause logic that used to live only in VSnackbar is no longer duplicated. Pausing while hovered or focused also satisfies WCAG 2.2.1 (Timing Adjustable). VSnackbar keeps driving its visual countdown timer through the composable's onStart/onClear hooks, so its behavior is unchanged.

Unit tests cover auto-dismiss, the pause-on-hover case, and the persistent default. The API description and a docs example have been added.

resolves #20409

#22952 addresses the same issue. This PR was opened first and also covers the requested shared composable, the hover/focus pause, and tests for both VSnackbar and VAlert. Happy to consolidate into whichever the team prefers.

Markup:

<template>
  <v-btn @click="visible = true">Show alert</v-btn>
  <v-alert
    v-model="visible"
    :timeout="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.

[J-Sek]

Can split that out separately if you want the two unified.

I think it would make sense. Including prop rename to timeout so we don't cause cognitive load by inconsistency when both props deliver basically the same behavior.

[eldardada]

@J-Sek updated. Renamed the prop to timeout and moved the shared auto-dismiss + hover/focus pause into a useAutoDismiss composable used by both VSnackbar and VAlert. Ready for another look when you have a moment, thanks!