Chat: hide tool-only rows with no text, surface trace on the next text reply

[dnviti]

Problem / Motivation

When the agent takes actions (running commands, calling tools, etc.) without producing any accompanying written reply, the chat transcript currently shows a bare row for that step anyway — an entry with no text at all, just an icon and a small "work" indicator. This clutters the conversation with rows that carry no readable content, making it harder to scan the chat for what the agent actually said.

The detailed record of what tools ran already exists in the trace view, so this information isn't lost — it's just being duplicated as an empty-looking row in the main conversation.

Goal

The chat should only show rows that have actual written content. Steps that consist purely of tool activity (no text) should stay out of the main conversation and live only in the trace. As soon as a text reply does arrive for that stretch of work, the chat should show a control on that message letting the user open the trace to review everything that happened leading up to it.

Scope

• Suppress chat rows that contain no text — i.e., steps that are pure tool/action activity with no written reply.
• Ensure that activity is still fully visible in the trace view, exactly as it is today.
• When a text reply follows one or more of these silent steps, attach a "view trace" control to that reply so the user can see everything that led up to it, not just the activity tied to that specific reply.
• Applies to both live/streaming responses and previously completed conversations viewed later.

Non-goals

• Changing how the trace view itself displays or organizes activity.
• Collapsing or folding entire conversation turns (tracked separately).
• Hiding tool activity from the trace — it must remain fully visible there.

Acceptance Criteria

• A step that produces only tool/action activity and no text does not appear as its own row in the main chat.
• All suppressed activity remains fully visible and reachable in the trace view.
• When a text reply arrives after one or more silent steps, the reply's row includes a control to view the trace, and that control surfaces the activity from the preceding silent steps as well as its own.
• This behaves correctly both while a response is still streaming in and when reviewing a past conversation.