copilot-cli ignores grant_types_supported and attempts authorization_code flow for client_credentials-only MCP servers

### Describe the bug

Hello. I am trying to set up an corporate MCP server in copilot. This MCP server is protected using an OAuth authorization server that advertises only client_credentials in `grant_types_supported`
Copilot-cli still attempts to initiate an interactive `authorization_code` flow instead.

Expected behavior:

copilot-cli should check [grant_types_supported] from the OAuth server metadata
If only client_credentials is listed, it should use that flow
It should NOT launch a browser for interactive auth

Actual behavior:

Launches a browser window for authorization code flow
This fails because the MCP server only permits client_credentials
Even when explicitly configured with "oauthGrantType": "client_credentials", oauthPublicClient": false, and "oauthClientId" in the config file as per https://docs.github.com/en/copilot/reference/copilot-cli-reference/cli-command-reference#headless-oauth-client_credentials-grant, the browser still launches

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Commencez par les chemins de configuration OAuth et de gestion des métadonnées de copilot-cli, en vous concentrant sur grant_types_supported ainsi que sur les paramètres oauthGrantType, oauthPublicClient et oauthClientId. Reproduisez le problème avec un serveur MCP qui annonce uniquement client_credentials, puis vérifiez que la CLI sélectionne le flux pris en charge et ne lance pas de navigateur.

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