fix(VBadge): make wrapper element use tag prop for valid HTML nesting (Fixes #23093)

[waterWang]

Summary

When VBadge is used inside VBtn (as shown in the VBadge documentation), the rendered HTML is invalid because VBadge hardcodes <div class="v-badge__wrapper"> inside VBtn's <span class="v-btn__content"> wrapper. The W3C validator reports:

Element "div" not allowed as child of element "span" in this context.

Fix

Make the v-badge__wrapper element use the tag prop instead of being hardcoded as a <div>. When tag="span" (or any phrasing-valid tag) is passed, both the root element and the wrapper element produce valid phrasing content.

Before (invalid HTML)

<span class="v-btn__content">
  <div class="v-badge">
    <div class="v-badge__wrapper">   <!-- INVALID: div inside span -->

After (valid HTML with tag="span")

<span class="v-btn__content">
  <span class="v-badge">
    <span class="v-badge__wrapper">  <!-- VALID: span inside span -->

Backward compatibility

Default tag="div" produces identical output to before: div > div > ...

Fixes #23093

[J-Sek]

This would be fine if we are convinced tag on <v-badge> is never used in real life. Otherwise it is too risky

[J-Sek]

We have badges within buttons and tabs across handful of examples. Assuming we don't bail on the risk of regression, those should be updated in the same PR.

[BatLeDev]

The three examples with a badge inside a button or a tab, for the update @J-Sek asked for — I checked every <v-badge> nested in a <v-btn>/<v-tab> across packages/docs/src, there are exactly these:

• examples/v-badge/prop-content.vue (3 badges, inside v-btn stacked)
• examples/v-badge/prop-dot.vue (2 badges, inside v-btn stacked)
• examples/v-badge/misc-tabs.vue (3 badges, inside v-tab)

All eight wrap only a v-icon or plain text, so adding tag="span" makes them valid with no other change.

Worth noting for anyone tempted to change the default instead: it would not work. examples/v-badge/misc-customization.vue wraps a v-avatar (a div), so a span default would just move the invalidity there. Keeping tag="div" as the default and opting in to span inside phrasing contexts is the right call.

The same defect on VChip is now tracked separately in #23103, with a fix proposed in #23106 — there it needs no tag discussion, since the chip root is already a span.

@waterWang I have those three example updates ready in waterWang#1 — a single commit on top of this PR's branch, so merging it into fix/23093-vbadge-wrapper-tag lands everything here in one piece.