fix: use absolute path for coverage.py executable

[sentry]

This PR addresses the FileNotFoundError: [Errno 2] No such file or directory: 'coverage' that occurs when the pycoverage plugin attempts to run coverage.py commands via subprocess.run.

The root cause was identified as a discrepancy between the PATH environment variable available to the main process (where shutil.which("coverage") successfully finds the executable) and the PATH inherited by the subprocesses spawned by subprocess.run. This can happen in various environments, particularly in CI/CD where virtual environments or shell configurations might not propagate correctly to subprocesses.

The fix involves capturing the absolute path of the coverage executable using shutil.which once at the beginning of the run_preparation method. This absolute path is then passed to and used by all subsequent subprocess.run calls within _generate_XML_report and _generate_JSON_report. This ensures that the exact, verified executable is invoked, regardless of the subprocess's inherited PATH.

Fixes CLI-8W