Function parse_coverage_json 

pub fn parse_coverage_json(
    json_str: &str,
) -> Result<HashMap<String, FileCoverage>>

Parse a coverage.json (pytest-cov JSON format) into a file-level coverage map.

Expected format:

{
  "files": {
    "brain/utils.py": {
      "summary": { "covered_lines": 42, "missing_lines": 8, "percent_covered": 84.0 }
    }
  }
}