When a plan generate by the plan tool fully completes, there's no need
for that to be in the activity bar anymore. It's complete and in the
next turn, the agent may come up with another plan and the cycle
restarts. So, this PR moves a fully complete plan to the thread view, so
that it stays as part of a given turn:

<img width="600" height="1858" alt="image"
src="https://github.com/user-attachments/assets/43ad4eb0-49d0-488c-bbbf-ab7956c1dd5a"
/>

The way this PR does this is by adding a new entry to `AgentThreadEntry`
and snapshotting the completed plan so we can display it properly in the
thread.

Release Notes:

- N/A