fix: only set AuthorizationServer when gh-host is explicitly configured

When gh-host is not set (github.com users), the AuthorizationServer
field was being set to just "login/oauth" (empty host + path), breaking
OAuth metadata. Now it only overrides the default when gh-host is
provided (GHES users), allowing github.com users to get the correct
default of "https://github.com/login/oauth".

Co-authored-by: atharva1051 <53966412+atharva1051@users.noreply.github.com>

Author: Copilot

pkg/http/server.go

@@ -126,7 +126,9 @@ func RunHTTPServer(cfg ServerConfig) error {
 	oauthCfg := &oauth.Config{
 		BaseURL:      cfg.BaseURL,
 		ResourcePath: cfg.ResourcePath,
-		AuthorizationServer: cfg.Host + "login/oauth",
+	}
+	if cfg.Host != "" {
+		oauthCfg.AuthorizationServer = cfg.Host + "login/oauth"
 	}
 
 	serverOptions := []HandlerOption{}