Add Django CI jobs for DjangoMessage tests

[lavr]

Summary

• Add pytest.mark.django marker to Django integration tests
• Use pytest.importorskip("django") to prevent collection errors when Django is not installed
• Add 3 dedicated CI jobs testing against Django 4.2, 5.2, and 6.0 (Python 3.12)
• Add tox envs django42, django52, django60 with pinned versions
• Exclude Django tests from regular unit jobs via -m "not e2e and not django"

Closes #201

Test plan

• django / 4.2 CI job passes
• django / 5.2 CI job passes
• django / 6.0 CI job passes
• Existing unit/e2e/typecheck jobs unaffected