feat(ci): enhance core testing workflow and introduce unified test command

- Refactored GitHub Actions workflow to support multiple OS environments (macOS, Linux, Windows) using a matrix strategy.
- Added a new `test-core` command in `xtask` for running all core integration tests with progress tracking and result summaries.
- Updated caching keys in the CI workflow to use OS-specific identifiers for better cache management.
- Consolidated test execution logic into a single source of truth in `xtask/src/test_core.rs`, ensuring consistency between CI and local runs.
- Improved documentation in `xtask/README.md` to reflect the new testing command and its features.