[Windows port] 3/8: Add Windows to CI and skip bash-only tests

[wmaciel]

Part of #7. Phase 3 of 8.

Add windows-latest to the pytest matrix. Tests that spawn .sh scripts get a requires_bash marker and skip on Windows; each is removed as its script is ported in later phases.

Changes: tests.yml matrix, pytest.ini marker, pytestmark on bash-spawning test modules.
Acceptance: CI green on ubuntu-latest and windows-latest.
Depends on: Phase 1 and Phase 2.

Drafted with Copilot 🤖

[chisingh]

Closed by #26