Guest accounts can't authenticate to specific tenant for MCP servers #5157

Describe the bug

The /mcp auth command does not allow us to specify a specific Entra ID tenant. This applies in particular to various MSFT Fabric related MCP servers (fabric-core, FabriIQ, etc.). When we are guests in a tenant, /mcp auth will only authenticate and use our home tenant. This issue also relates to what is configurable in various MCP JSON config files. We need a TenantID element - OR the ability to specify one at runtime. Note that this is separate from our github/copilot login and auth. We could have different tenants for that vs our MCP servers.

Affected version

GitHub Copilot CLI 1.0.98.

Steps to reproduce the behavior

1. Start copilot CLI
2. Type: /mcp auth
3. Note no argument to specify tenant. Auth flow doe not allow us to login to a specific tenant - just the home tenant.

Expected behavior

/mcp auth <server> [--tenantId <tenantId>]

Additional context

• Microsoft Windows 10.0.22631 (Windows 11)
• PowerShell 7.6.6