Markdown link renderer and Thread View Mention links

[pkondzior]

This PR adds native-looking Mentions in the Agent Panel Thread view by introducing a custom inline Markdown link renderer.

Dependency

• This work depends heavily on Introduce GPUI inline element #48057, because Mentions require the new inline layout system that can wrap arbitrary UI elements inside flowing text.
• Because that dependency does not live in the Zed repo, I can’t reasonably open this PR with a base branch outside of zed-industries/zed

Demo:

Mentions.Demo.1.mp4

Also included

• This PR also includes Make mention creases actionable #48054. I extracted the MentionCrease changes into this MVP PR.
• The MentionCrease changes are independent: they do not depend on the Markdown link renderer or the gpui inline layout work.

List of changes:

• Added a custom markdown link rendering pipeline via LinkRenderer and LinkRenderContext, allowing callers to render links as arbitrary elements instead of plain inline text.
• Introduced LinkLabel to preserve mixed inline content (text + child elements) inside links, so custom renderers can reproduce the full link label.
• Reworked the markdown inline builder to track nested inline content using PendingInline/PendingItem and a link_stack, then render links at close time via pop_link.
• MarkdownElement::link_renderer(...) now lets the Agent thread view provide a custom renderer; mention links are rendered as MentionCrease with active/selected styling.
• AcpThreadView::link_renderer(...) parses links into MentionUri and renders mentions as clickable buttons; non‑mention links fall back to the default link element.

Related #37563

Release Notes:

• Improved Agent Thread mention links render as Zed-style clickable buttons with tooltips, powered by a custom markdown link renderer for more consistent visuals and smoother navigation.

[benbrandt]

Thanks for this, but given the state of things and this PR's dependencies, going to close for now

[xyt-dev]

This is a very useful feature. Looking forward to seeing it implemented soon!