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)

Inizia con il flusso MCP Settings e la configurazione plugin .mcp.json, quindi riproduci il comportamento dell’endpoint Atlassian descritto nell’issue. Confronta il relativo rilevamento e la gestione dell’autenticazione con LeanIX e Lucid; è completato quando Sign in apre il flusso OAuth nel browser, viene salvato un token e gli strumenti Atlassian vengono caricati nelle sessioni.

Scritto dal modello di indicizzazione a partire dal testo della issue.