fix(cli): improve error message display for object errors

Previously, when a tool execution failed with an error object (not an
Error instance), getErrorMessage() would return '[object Object]',
hiding useful error information from users.

This change improves getErrorMessage() to:
1. Extract the 'message' property from error-like objects
2. JSON.stringify plain objects to show their full content
3. Fall back to String() only when JSON.stringify fails

Fixes #1338

packages/cli/src/utils/errors.test.ts

@@ -105,9 +105,25 @@ describe('errors', () => {
       expect(getErrorMessage(undefined)).toBe('undefined');
     });
 
-    it('should handle objects', () => {
-      const obj = { message: 'test' };
-      expect(getErrorMessage(obj)).toBe('[object Object]');
+    it('should extract message from error-like objects', () => {
+      const obj = { message: 'test error message' };
+      expect(getErrorMessage(obj)).toBe('test error message');
+    });
+
+    it('should stringify plain objects without message property', () => {
+      const obj = { code: 500, details: 'internal error' };
+      expect(getErrorMessage(obj)).toBe(
+        '{"code":500,"details":"internal error"}',
+      );
+    });
+
+    it('should handle empty objects', () => {
+      expect(getErrorMessage({})).toBe('{}');
+    });
+
+    it('should handle objects with non-string message property', () => {
+      const obj = { message: 123 };
+      expect(getErrorMessage(obj)).toBe('{"message":123}');
     });
   });
 

packages/cli/src/utils/errors.ts

@@ -17,6 +17,27 @@ export function getErrorMessage(error: unknown): string {
   if (error instanceof Error) {
     return error.message;
   }
+
+  // Handle objects with message property (error-like objects)
+  if (
+    error !== null &&
+    typeof error === 'object' &&
+    'message' in error &&
+    typeof (error as { message: unknown }).message === 'string'
+  ) {
+    return (error as { message: string }).message;
+  }
+
+  // Handle plain objects by stringifying them
+  if (error !== null && typeof error === 'object') {
+    try {
+      return JSON.stringify(error);
+    } catch {
+      // If JSON.stringify fails (circular reference, etc.), fall back to String
+      return String(error);
+    }
+  }
+
   return String(error);
 }