fix(oauth): preserve GHEC tenant domains in OAuth endpoints

For ghe.com (GHEC), keep the full tenant domain (mycompany.ghe.com)
instead of incorrectly stripping to base domain (ghe.com).
Only strip api. prefix for api.github.com → github.com.

Author: SamMorrowDrums

internal/oauth/oauth.go

@@ -518,13 +518,10 @@ func getOAuthEndpoints(host string) (authURL, tokenURL, deviceAuthURL string) {
 		hostname = hostname[:idx]
 	}
 
-	// For github.com or ghe.com (GHEC), use the main domain
-	if strings.HasSuffix(hostname, "github.com") || strings.HasSuffix(hostname, "ghe.com") {
-		// Extract base domain (e.g., github.com from api.github.com)
-		parts := strings.Split(hostname, ".")
-		if len(parts) >= 2 {
-			hostname = parts[len(parts)-2] + "." + parts[len(parts)-1]
-		}
+	// For github.com, strip api. subdomain (api.github.com → github.com)
+	// For ghe.com (GHEC), keep the full tenant domain (mycompany.ghe.com stays as-is)
+	if hostname == "api.github.com" {
+		hostname = "github.com"
 	}
 
 	authURL = fmt.Sprintf("%s://%s/login/oauth/authorize", scheme, hostname)