a11y: navbar and sidebar tools declared with only an `icon:` have no accessible name

The documented way to declare navbar and sidebar tools uses icon: alone, with no text:. That renders links with an empty accessible name, which axe-core reports as link-name.

Taking the example straight from https://quarto.org/docs/websites/website-navigation.html#navbar-tools:

project:
  type: website

website:
  title: "Tools repro"
  navbar:
    left:
      - href: index.qmd
        text: Home
    tools:
      - icon: bluesky
        href: https://bsky.app
      - icon: github
        menu:
          - text: Source Code
            href: https://code.com
          - text: Report a Bug
            href: https://bugs.com

format:
  html:
    axe:
      output: json

produces

<a href="https://bsky.app" title="" class="quarto-navigation-tool px-1" aria-label=""><i class="bi bi-bluesky"></i></a>
<a href="" title="" id="quarto-navigation-tool-dropdown-0" class="quarto-navigation-tool dropdown-toggle px-1" data-bs-toggle="dropdown" aria-expanded="false" role="link" aria-label=""><i class="bi bi-github"></i></a>

The icon is drawn by a class on an empty <i>, so there is no text node to fall back on. Chrome's accessibility tree announces both as link "", and axe-core reports link-name (serious, wcag2a / wcag244 / wcag412) on three nodes for that config: the navbar tool, the navbar dropdown tool, and the sidebar tool. Sidebar tools go through the same template, so website: sidebar: tools: is affected the same way, and the sidebar example in the docs has the same shape.

aria-label="" is worse than no attribute at all, since an empty aria-label blanks the name rather than letting anything else supply one. Both attributes are interpolated from tool.text, which is unset here:

https://github.com/quarto-dev/quarto-cli/blob/2117a09798cba666b531b76642ee154e201b7d5c/src/resources/projects/website/templates/navtools.ejs#L26-L46

Authors have little reason to set text: on a tool, since it never renders as visible text, only as the title tooltip. aria-label: is accepted on the item and does fix it, but nothing points authors there. Note the same lines already avoid emitting empty rel and target, so title and aria-label look like they were just missed in that pass.

We could omit both attributes when there is no value, and fall back to a name derived from the icon when neither text: nor aria-label: is given, so that the documented configuration produces a named link by default.

Related: #14380 and #14655 are the same axe rule on different elements, and #14373 collects a batch of axe-core findings on Quarto sites.

cc @cwickham as I don't know if you spotted this one, but 🤖 reported it to me while testing

Start with src/resources/projects/website/templates/navtools.ejs at the linked lines, then compare the navbar and sidebar tool examples in the website navigation documentation. Reproduce the icon-only configuration with the shown axe output and verify that the resulting links have meaningful accessible names without empty title or aria-label attributes; done means the link-name violations are gone for navbar and sidebar tools.