Allow button directives to use Markdown link references

[Copilot]

:::{button} only accepted inline Markdown links ([text](url)), rejecting valid reference-style links such as [text][ref]. This updates button directive validation to allow both forms so referenced links render the same as direct links.

• Directive validation

  • Broadened ButtonBlock link validation from inline-only links to a single Markdown link in either form:
    • [text](url)
    • [text][ref]
  • Kept the existing guardrails around non-link content, multiple links, and nested blocks.

• Error guidance

  • Updated button directive error messages to show both supported link forms, so failures point users at the accepted syntax.

• Regression coverage

  • Added a focused button directive test for a reference-style link with a trailing link definition to ensure the directive accepts and renders it.

:::{button}
[Open][kibana-url]
:::

[kibana-url]: <https://foo.example.com>

[theletterf]

@copilot, also address this failing case:

:::{button}
[Logs](https://example.kb.us-west2.gcp.elastic-cloud.com/app/discover#/?_g=(filters:!(),refreshInterval:(pause:!t,value:60000),time:(from:now-15h,to:now))&_a=(columns:!(),dataSource:(type:esql),filters:!(),interval:auto,query:(esql:'FROM%20logs*,-logstash*,filebeat-*%20%7C%20WHERE%20service.name%20%3D%3D%20%22apm-oblt%22'),sort:!())&_tab=(tabId:'43c2b949-f679-43a8-bf68-9bf209c2fc5a'))
:::

If not possible, explain why.

[Copilot]

@copilot, also address this failing case:

...

I didn’t make a code change for this one because the sample isn’t a single valid link. The " in filters:!("> closes the href early, so the parser/browser see <a href="...filters:!(">Logs</a> plus trailing text outside the anchor. To make it valid, that quote needs to be encoded (%22 or "), and the safest option is to use Markdown link syntax (inline or reference-style) instead of raw HTML for this URL.