fix: close span tag in copy button HTML

[corazzione]

Summary

The copy button in htmlCodeBlock.ts had a malformed HTML — the closing <span> tag was written as <span> instead of </span>, leaving the inner span element unclosed and opening a new unclosed span before </button>.

Before:

<button class="copy-button" data-copy-content>
  <span class="codicon codicon-copy" title="Copy type to clipboard">
  <span>  <!-- ← wrong: opens a new span instead of closing -->
</button>

After:

<button class="copy-button" data-copy-content>
  <span class="codicon codicon-copy" title="Copy type to clipboard"></span>
</button>

Test plan

• All 33 tests pass (packages/formatter: 27, packages/vscode-formatter: 6)
• TypeScript clean on both packages after workspace deps are built

[yoavbls]

Thanks!