Simplify and modernize integration test code
- Use http.StatusOK instead of magic number 200
- Use io.ReadAll instead of manual buffer loop in harness
- Use strings.NewReader instead of bytes.NewBufferString
- Use testing.Short() instead of COG_TEST_FAST env var in pty tests
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
