agent: Add expand_code_block setting

[cachebag]

Objective

Addresses #58333.

Long ACP agent turns are hard to scan because verbose blocks push the prose apart. Diff and terminal cards are already configurable via expand_edit_card / expand_terminal_card, but fenced code blocks inside agent messages had no collapse at all.

Code-heavy responses therefore dominated the thread with no settings-only workaround and this has been a really frustrating for many users, including me, as it makes a lot of the agent output borderline unreadable.

Solution

Adds agent.expand_code_block (this is defaulted to true).

When set to false, code blocks in agent messages render as a single row showing the language and line count (for example rust · 42 lines). Clicking the row expands the block, and the hover toolbar on an expanded block gains a collapse button next to the existing wrap and copy buttons. A block that contains the active in-thread search match stays open so navigating between matches never lands on hidden text.

• markdown: new MarkdownElement::collapse_code_blocks option. Collapsed blocks skip content layout entirely and render a summary row instead. Per-block expanded state lives on the Markdown entity, keyed by source offset like the existing wrapped_code_blocks, so it survives re-renders and streaming appends.
• settings_content / agent_settings / default.json / settings UI: the new setting, alongside expand_edit_card and expand_terminal_card.
• agent_ui: applies the setting to assistant message content only. Tool output and compaction summaries are already collapsible as a whole and are left unchanged.

Testing

Ideally, some candidates from #58333 can also take a look and try it for themselves and post screenshots

Here are some code blocks with the setting on:

And with the setting disabled:

I wrote and tested this patch on macOS and tested on Linux but not on Windows

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• Added agent.expand_code_block setting to have code blocks in agent messages start collapsed, expandable individually.