MCP Oauth fails when the server metadata URL is redirected

Describe the bug

When the .well-known/openid-configuration is behind an http redirect, authentication fails with:

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

I believe that copilot-cli checks the issuer against the original URL instead of the redirected URL. Note that RFC 8414 states that the issuer must match "the URL used to retrieve the metadata," which strictly speaking is the redirection target, not the source.

Affected version

GitHub Copilot CLI 1.0.83.

Steps to reproduce the behavior

1. Run an MCP server on a different server from its OAuth authorization server, e.g. https://my.mcp.server vs https://my.oauth.server
2. Configure the MCP server so that https://my.mcp.server/.well-known/openid-configuration returns an HTTP 301 redirect to https://my.oauth.server/.well-known/openid-configuration
3. Configure the OAuth server so that https://my.oauth.server/.well-known/openid-configuration specifies https://my.oauth.server as the issuer
4. In copilot-cli, type /mcp add FooBar, select type HTTP, remote server URL https://my.mcp.server, and the client ID when asked

Expected behavior

Copilot should ask to authenticate, then open the authentication URL in my browser (or display the authentication code and URL so that I can copy-paste them in my browser), then complete the authentication flow.

Additional context

This may be related to #4480 except that in #4480, the Atlassian MCP server gave the wrong issuer, whereas here the issuer is correct for the real URL and copilot-cli is checking against the wrong URL.

Reproduziere den Fehler, indem du dafür sorgst, dass der .well-known/openid-configuration-Endpunkt des MCP-Servers eine 301-Weiterleitung zurückgibt, und verfolge anschließend die Issuer-Validierung im /mcp add HTTP-Flow. Erledigt ist die Aufgabe, wenn Metadaten von der weitergeleiteten URL akzeptiert werden, sobald ihr Issuer mit dieser finalen URL übereinstimmt, und die Authentifizierung bis zum Browser oder zum angezeigten Autorisierungscode fortschreitet.