ci: add unit test coverage support

### Current Limitation

Currently, our CI workflows only run unit tests but do not track or report code coverage.
This makes it difficult to monitor how well our tests cover the codebase and identify areas lacking test coverage.

### Suggested Improvement

Integrate code coverage reporting into the CI pipeline using [Codecov](https://about.codecov.io/).
- Add coverage reporting configuration for all SDK packages.
- Upload coverage reports to Codecov after CI runs.
- Enforce minimum coverage thresholds (optional) to maintain quality.
- Provide a coverage badge in the repository README for visibility.

### Please select the package issue is related to

@asgardeo/express, @asgardeo/browser, @asgardeo/javascript, @asgardeo/nextjs, @asgardeo/nuxt, @asgardeo/node,
@asgardeo/react, @asgardeo/react-router, @asgardeo/vue

### 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.