fix(cli): resolve environment loading and auth validation issues in ACP mode (#18025)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/core/src/utils/errors.ts

@@ -15,11 +15,41 @@ export function isNodeError(error: unknown): error is NodeJS.ErrnoException {
 }
 
 export function getErrorMessage(error: unknown): string {
-  if (error instanceof Error) {
-    return error.message;
+  const friendlyError = toFriendlyError(error);
+  return extractMessage(friendlyError);
+}
+
+function extractMessage(input: unknown): string {
+  if (input instanceof Error) {
+    return extractMessage(input.message);
   }
+
+  if (typeof input === 'string') {
+    const trimmed = input.trim();
+    // Attempt to parse JSON error responses (common in Google API errors)
+    if (
+      (trimmed.startsWith('{') && trimmed.endsWith('}')) ||
+      (trimmed.startsWith('[') && trimmed.endsWith(']'))
+    ) {
+      try {
+        const parsed = JSON.parse(trimmed);
+        const next =
+          parsed?.error?.message ||
+          parsed?.[0]?.error?.message ||
+          parsed?.message;
+
+        if (next && next !== input) {
+          return extractMessage(next);
+        }
+      } catch {
+        // Fall back to original string if parsing fails
+      }
+    }
+    return input;
+  }
+
   try {
-    return String(error);
+    return String(input);
   } catch {
     return 'Failed to get error details';
   }