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)

Commencez par le flux MCP Settings et la configuration plugin .mcp.json, puis reproduisez le comportement du endpoint Atlassian décrit dans l’issue. Comparez sa découverte et sa gestion de l’authentification avec LeanIX et Lucid ; c’est terminé lorsque Sign in ouvre le flux OAuth dans le navigateur, qu’un token est enregistré et que les outils Atlassian se chargent dans les sessions.

Rédigé par le modèle d'indexation à partir du texte de l'issue.