Dev Container session creation lacks visible progress

Testing #335485

Creating a new Agent Host session in a Dev Container can take a long time, but the new-session UI provides little or no meaningful progress while the container is being prepared.

## Reproduction

1. Enable Agent Host Dev Container support.
2. Select a folder with a Dev Container configuration.
3. Choose **Use Dev Container**.
4. Enter a prompt and start the session.
5. Wait while the Dev Container is created and connected.

## Actual

The new-session UI appears stuck for a long time. It is not clear whether the operation is still making progress, waiting for input, or has failed.

The session eventually gets created successfully, and the Dev Container Output channel contains the underlying progress, but users must know to look there.

## Expected

Show clear, ongoing progress in the new-session UI while preparing the Dev Container. The progress should communicate the current phase, such as:

- Starting/building the Dev Container
- Connecting to the container
- Initializing the Agent Host session
- Waiting for the session to become ready

Provide a clear failure or actionable link to logs if startup stalls or fails.