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.

まず @asgardeo/browser パッケージの最新のカバレッジを確認し、既存のユニットテストスイートを Vitest で実行して、テストされていない行と分岐を特定します。特定の関数、メソッド、またはコンポーネントを選択し、通常の入力、境界値、無効な入力に対するテストを追加します。テストが成功し、選択したパスがカバーされれば完了です。

索引モデルが issue の本文から書いたものです。