Use reusable workflow to eliminate setup duplication between test and collateral

[Copilot]

test and collateral jobs duplicated setup steps, making it easy to add a workaround to one but forget the other — critical because diffcov in collateral actually runs the tests and needs identical setup.

Changes

• New .github/workflows/test-suite.yml — reusable workflow_call workflow encapsulating all shared logic:

  • Inputs: python (default 3.x), platform (default ubuntu-latest), tox-env (default empty)
  • fetch-depth: 0 (required for diffcov)
  • Conditional system dep install for Python 3.15/ubuntu (now applies to both test and diffcov automatically)
  • Setup Python → install tox → run tox or tox -e <env> based on tox-env input
  • Shared env vars (FORCE_COLOR, TOX_OVERRIDE, etc.)

• Updated main.yml — both test and collateral now delegate to the reusable workflow:

test:
  strategy:
    matrix: { python: [...], platform: [...] }
  uses: ./.github/workflows/test-suite.yml
  with:
    python: ${{ matrix.python }}
    platform: ${{ matrix.platform }}
  continue-on-error: ${{ matrix.python == '3.15' }}

collateral:
  strategy:
    matrix: { job: [diffcov, docs] }
  uses: ./.github/workflows/test-suite.yml
  with:
    platform: ubuntu-latest
    tox-env: ${{ matrix.job }}

[webknjaz]

Try experimenting with integrating https://github.com/ansible/ansible-builder/blob/e94e544b9a003040efde05023ea755d01ee302d9/.github/workflows/ci.yml#L80-L154 in some canary project to see how it goes.