MCP OAuth: 'needs authentication' link never launches any visible auth flow (Atlassian/Jira, macOS, CLI 1.0.83)

Describe the bug

The Atlassian (Rovo) MCP server repeatedly shows a "Jira MCP needs authentication" prompt, but clicking the provided authentication link — or attempting to authenticate manually — never launches any visible auth flow. No browser window/tab opens, no OAuth consent page appears, and nothing surfaces in the UI. The prompt simply reappears, so the server can never be authenticated and its tools are never usable.

This is the browser-launch / auth-hand-off side of MCP OAuth, not the token-bridging bug in #4096 (which is fixed on my version). It looks related to #4400 (browser login URL wrapping/fallback) and #3130 (unable to open browser for auth).

Affected version

GitHub Copilot CLI 1.0.83 (bundled engine used by the Copilot desktop app "agents" window).

Environment

• macOS (Apple Silicon)
• The failing flow is in the app / agents window, which manages its own MCP configuration.
• The standalone terminal CLI has no MCP servers configured (~/.copilot/mcp-config.json = {"mcpServers":{}}, copilot mcp list = "No MCP servers configured", and ~/.copilot/mcp-oauth-config/ does not exist). So the Atlassian server exists only on the app side, and it is the app-side "authenticate" link that dead-ends.

Steps to reproduce the behavior

1. In the Copilot app (agents window), add/enable the Atlassian Remote MCP server (https://mcp.atlassian.com/v1/mcp).
2. When prompted with "Jira MCP needs authentication", click the provided authentication link (or choose to authenticate manually).
3. Observe: no browser opens, no OAuth consent page appears, nothing visible happens.
4. The "needs authentication" prompt returns; the server never becomes authenticated and no atlassian-* tools are exposed.

Expected behavior

Clicking the authentication link should open the system default browser to the OAuth consent URL (or, if the browser cannot be launched, print/copy the raw URL so it can be opened manually) and complete the callback so the server authenticates.

Additional notes

• This consistently blocks my workflow — the Atlassian/Jira MCP tools are never available because auth can never be completed.
• A raw-URL fallback (print the login URL to stdout / provide a copyable link) would at minimum unblock manual authentication when auto-launch fails.

Reproduce the flow in the Copilot app agents window with https://mcp.atlassian.com/v1/mcp enabled, then compare it with the standalone CLI state described by ~/.copilot/mcp-config.json, copilot mcp list, and ~/.copilot/mcp-oauth-config/. Trace the authentication-link hand-off and browser-launch entry point; done means the link opens the system browser or exposes a copyable raw URL, and the Atlassian tools become available after callback.