Agent Host steering starts a new turn instead of steering the active turn

- Copilot Chat Extension Version: Built-in Agent Host
- VS Code Version: Code - Insiders
- OS Version: macOS
- Feature: Agents Window steering during an active turn
- Selected model: gpt-5.6-sol
- Logs: Agent Host debug-log bundle available privately

Steps to Reproduce:

1. Start an agent turn in the Agents Window.
2. While the agent is still responding, enter another instruction and submit it as steering.

Expected:

The new instruction steers the active agent turn.

Actual:

The active answer continues, and the steering instruction is handled as a separate follow-up turn, making it appear concatenated with the original answer rather than steering it.

Additional diagnostics:

The debug logs show the `chat/pendingMessageSet` action was correctly marked as `kind: steering`, but the SDK recorded the message with `delivery: idle` and started a separate turn.