Atlassian MCP server auto-connects without completing OAuth flow (Sign in button disappears)

## Description

The Atlassian MCP server (https://mcp.atlassian.com/v1/mcp) shows a green checkmark (connected) in MCP Settings without completing the OAuth browser flow. No atlassian-* tools are loaded in sessions.

## Expected behavior

Clicking 'Sign in' or toggling the server on should open a browser for OAuth authorization (like LeanIX and Lucid do).

## Actual behavior

- The 'Sign in' button flashes briefly and disappears
- Green checkmark appears immediately without browser redirect
- No OAuth token is saved (no .tokens.json file in ~/.copilot/mcp-oauth-config/)
- Sessions do not have access to any Atlassian tools

## Steps to reproduce

1. Add Atlassian MCP to a plugin .mcp.json:
```
"atlassian": { "type": "http", "url": "https://mcp.atlassian.com/v1/mcp" }
```
2. Open MCP Settings
3. Toggle Atlassian on
4. Observe: green checkmark appears without browser opening

## Root cause hypothesis

Atlassian MCP endpoint responds to discovery/health-check requests with HTTP 200 without requiring authentication. The app interprets this as 'connected' and skips the OAuth flow. Other MCP servers (LeanIX, Lucid) correctly require auth on these requests.

## Workaround

Using Atlassian REST API directly with personal API tokens.

## Environment

- Copilot CLI version: 1.0.69
- OS: Windows 11
- Plugin: custom plugin with Atlassian MCP config
- Other MCP servers working correctly: LeanIX, Lucid, QMD (local)

先从 MCP Settings 流程和 plugin .mcp.json 配置开始，然后复现 issue 中描述的 Atlassian 端点行为。将其发现机制和身份验证处理与 LeanIX 和 Lucid 进行比较；当 Sign in 打开 OAuth 浏览器流程、保存 token，并且 Atlassian 工具在会话中加载时，即表示完成。

由索引模型根据 Issue 内容生成。