feat: collapse long chat messages with an expand control #2303

Problem

Long messages in the chat transcript can take up a large amount of vertical space. Add a maximum number of visible lines for long messages, with an affordance to expand and collapse the full content. In the collapsed state, show only the configured/defined line limit and make it clear that more text is available.

Expected behavior

• Messages longer than the line limit are initially truncated/collapsed in the transcript.
• Provide an obvious “Show more” / “Show less” (or equivalent) control to expand and collapse the message.
• Preserve the full message for copy/select, accessibility, and session history; collapsing must only affect presentation.
• Apply consistently to long chat messages in the relevant frontends (at minimum macOS; assess TUI behavior and consistency).
• Avoid changing short messages or disrupting streaming/replayed messages.

Acceptance criteria

• A long message displays no more than the chosen maximum number of lines while collapsed, with a clear expansion control.
• Expanded content can be collapsed again.
• Cover boundary cases around the line limit, very long messages, streaming updates, and transcript replay.