agent: Cap the height of large tool output and diff blocks

[ebertn]

Tool output (including MCP/context server results), file diffs, and other text content blocks in the agent thread were rendered at full height. An accidentally huge buffer could lay out thousands of lines at once and cause serious UI/scrolling performance problems.

This caps each such block and reveals the rest behind an expand control.

Behavior:

• Markdown/text output is capped at the new agent.tool_output_max_lines setting (default 10).
• File diffs are sized proportionally to their length (3 + lines / 10), so small diffs show almost fully and large diffs grow slowly.
• A centered, transparent chevron over a bottom vignette toggles each block between collapsed and expanded (2x the cap), and flips to reflect the state.
• Expanded blocks are scrollable (and still capped); collapsed blocks are static previews. Scrolling chains out to the thread at the block's top/bottom edges.
• agent.tool_output_max_lines = 0 disables capping entirely.

Implementation notes:

• Generalizes the existing thinking-block constrain pattern into a reusable render_constrained_output_block helper, gated on the content's line count (short outputs render unchanged). The gating logic is a pure function with unit tests.
• Scroll containment: the thread's list wheel handler only checks hit-test membership (not propagation), so the block is occluded to stop the thread from also scrolling, and we drive the block's scroll and hand off to list_state.scroll_by at the edges.
• Terminal output is already bounded (fixed-height scroll region + output truncation), so it's left as-is.

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

Release Notes:

• Added an agent.tool_output_max_lines setting (default 10) that caps the height of large tool output and diff blocks in the agent panel behind an expandable, scrollable view, so an oversized buffer no longer degrades UI performance.