MCP OAuth broken with agentgateway + Entra ID

Describe the bug

Earlier, https://github.com/github/copilot-cli/issues/4490 was closed but I tested with the 1.0.81 prerelease and for our case the same error persists:

Authentication failed: MCPOAuthError: Incompatible authorization server: authorization server advertised an issuer that does not match the URL its metadata was discovered from (RFC 8414 §3.3); refusing to connect

This issuer mismatch is introduced by https://github.com/agentgateway/agentgateway/pull/2408 that works around several MCP OAuth & Entra ID incompatibilities at the cost of introducing an authorization server discovery proxy.

This works fine with Copilot version 1.0.78. The error occurs with GitHub Copilot CLI 1.0.80 and GitHub Copilot CLI 1.0.81-8 preview.

Can you please implement a skipIssuerValidation configuration option to make this work again?

Affected version

GitHub Copilot CLI 1.0.81-8

Steps to reproduce the behavior

Try to authenticate to MCP server configured on agentgateway with Entra ID following the documentation.

The https://agentgateway.domain/.well-known/oauth-authorization-server/mcp resource returns the original Entra ID issuer:
"issuer": "https://login.microsoftonline.com/tenant-id/v2.0"

Expected behavior

Copilot can authenticate to MCP OAuth server with agentgateway authorization server proxy and Entra ID again.

Bắt đầu với luồng xác thực OAuth của Copilot CLI MCP và tái hiện lỗi trên thiết lập agentgateway Entra ID được mô tả trong issue. So sánh các phiên bản 1.0.78, 1.0.80 và 1.0.81-8, sau đó xác minh rằng xác thực thành công trong khi vẫn giữ nguyên hành vi an toàn của issuer của authorization-server.