chore: add unit test coverage for untested code paths in `@asgardeo/browser`

### Current Limitation

Some code paths and edge cases in @asgardeo/browser are not covered by unit tests, leading to reduced confidence in code changes, potential bugs, and lower test coverage.

- Reduced confidence in code changes and refactoring
- Potential undetected bugs in less common scenarios
- Lower overall test coverage metrics
- Increased manual testing effort
- Difficulty in maintaining code quality over time

Without adequate unit tests, the codebase may contain:

- Functions or methods that are not validated for all input variations
- Edge cases that may break silently
- Logic errors that are only caught in integration or production environments

### Suggested Improvement

Identify and add unit tests for untested code paths and edge cases in @asgardeo/browser. Improve test coverage and reliability.

## Implementation Plan

1. Review the latest code coverage to identify untested lines/branches.
2. Select a function, method, or component that lacks test coverage.
3. Write a new unit test that:
- Covers normal, boundary, and invalid input cases
- Asserts correct output and expected error handling
4. Run the test suite to confirm the new test is effective and passes.
5. Update documentation or test coverage badges as necessary.

**References:**
- https://vitest.dev/
- [Improving test coverage](https://blog.codecov.io/how-to-improve-code-coverage-ec7d5c5e1236/)

### Please select the package issue is related to

@asgardeo/browser

### Version

Current

### Reporter Checklist

- [x] I have searched the existing issues and this is not a duplicate.
- [x] I have provided all the necessary information.
- [x] I have verified the improvement is not available in the latest version of the package.

Bắt đầu bằng cách xem xét coverage mới nhất cho package @asgardeo/browser và chạy bộ unit test hiện có của package bằng Vitest để xác định các dòng và branch chưa được kiểm thử. Chọn một function, method hoặc component cụ thể, sau đó thêm các test cho input thông thường, input biên và input không hợp lệ; công việc được hoàn thành khi các test đều pass và các path đã chọn được cover.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.