Agentic lakehouse: session and trace observability

### Task Description

**What needs to be done:**
Durable logs of every request/session and per-turn traces (prompts, tool calls and results, latencies, token usage), with an exportable format; persistent chat history in the UI.

**Why this task is needed:**
Operators need to see what the agent did and why; users expect conversations to survive restarts.

### Task Type
Other

### Related Issues
**Parent feature issue:** #19256

Start by reading the parent feature issue #19256 and identify the existing agent and UI entry points for sessions and conversations. Done should include durable request and per-turn traces with an exportable format, plus chat history that survives restarts.

Written by the indexing model from the issue text.