[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**.

在 Copilot 应用中，从 New session 屏幕使用面板菜单 (+) 和 Extensions 重现该问题，然后将其与活动会话以及 issue #2862 中讨论的侧面板行为进行比较。完成的标准是 Azure DevOps extension canvas 可以在第一个 prompt 之前打开，以便先选择 work item。

由索引模型根据 Issue 内容生成。