[ci] Codecov project check fails with false coverage drop on single-module PRs

[jhagberg]

Describe the bug

PRs that only modify files in one Go module (e.g. sda/) cause the codecov/project status check to report a large coverage drop (~18%). This is a false negative — the actual code coverage is fine (codecov/patch passes), but Codecov sees fewer uploads on HEAD than on BASE and treats missing modules as zero coverage.

Root cause: all four test jobs in .github/workflows/test.yml upload to the same unittests flag without carryforward enabled. There is no codecov.yml config in the repo.

Steps to reproduce

1. Open a PR that only touches files in sda/ (e.g. PR feat(download): new v2 download service replacing sda-download #2187)
2. All CI test jobs run and upload coverage
3. codecov/project reports ~-18% coverage drop and fails
4. codecov/patch passes (all modified lines covered)

Expected behavior

• codecov/project should not fail when coverage for unchanged modules is unchanged
• Add codecov.yml with carryforward: true on the unittests flag (or split into per-module flags)

Additional context

Proposed minimal fix — add codecov.yml at repo root:

coverage:
  status:
    project:
      default:
        threshold: 1%
    patch:
      default:
        target: auto

flags:
  unittests:
    carryforward: true

Alternative: split into per-module flags (unittests-sda, unittests-sda-download, etc.) for more granular tracking.

Estimation of size

small

Estimation of priority

low