v6 produces empty coverage report

[dalnoki]

Upgrading from codecov/codecov-action@v5 to v6 causes Codecov to receive an empty report. The CI upload step reports success (exit 0, "Process Upload complete"), but Codecov's processing pipeline records:

Report is empty and cannot be processed

Downgrading back to v5 immediately resolves the issue with no other changes.

1. Use codecov/codecov-action@v5 — coverage uploads and processes correctly
2. Upgrade to codecov/codecov-action@v6
3. CI upload step exits 0 and logs "Process Upload complete"
4. Codecov dashboard shows "Unusable report" and breadcrumb trail shows "Report is empty and cannot be processed"

[linear]

CCMRG-2215