Add unit tests for open_stream() functionality and mock stream implementation

- Implemented a MockStream class for simulating stream behavior in tests.
- Added tests for ClientImpl::StreamHandle, including basic validity checks and error handling.
- Created BodyReader tests to validate reading from streams with and without errors.
- Developed OpenStreamTest suite to test various scenarios including basic requests, large responses, and chunked responses.
- Included SSL support tests for secure connections and chunked data handling.
- Enhanced test coverage for gzip, Brotli, and Zstd compression handling.

Author: yhirose