[AzureDevOpsExtension] Allow extension canvases on New session screen (before first prompt)

### Summary

I can use built-in panels (Changes, Terminal, Browser) on the **New session** screen, but I cannot open **extension canvases** there.

### Environment
App version 1.1.14
Windows 10.0.26200 Build 26200
azure-devops-copilot-plugin 0.1.53

### Current behavior

- I have the Azure DevOps plugin installed:

`https://github.com/microsoft/azure-devops-copilot-plugin`

- In an **active session/chat**, I can open the **Azure DevOps canvas** from Extensions.

- In the **New session** screen (before first prompt), extension canvas options are not available/usable and indicate an active session is required.

### Expected behavior

I should be able to open extension canvases (at least Azure DevOps) directly from the New session screen, so I can pick a work item first and then start the session based on it.

### Why this matters

A common workflow is selecting context before prompting. For Azure DevOps, this means choosing a work item first, then starting the coding session from that item.

### Repro steps

1. Install Azure DevOps Copilot plugin.

2. Open Copilot app and go to **New session**.

3. Open panel menu (`+`) → Extensions.

4. Observe extension canvas is not available/usable until a session is active.

5. Start/activate a session and open Extensions again.

6. Azure DevOps canvas can now be opened.

### Notes

Issue #2862 addressed side panel visibility on New session. This appears to be a separate gap specific to **extension canvas availability before first prompt**.

Reproduce the issue in the Copilot app from the New session screen using the panel menu (+) and Extensions, then compare it with an active session and the side-panel behavior discussed in issue #2862. Done means the Azure DevOps extension canvas can open before the first prompt so a work item can be selected first.

索引モデルが issue の本文から書いたものです。