ci: add backend unit tests to Tests workflow

[shivv23]

Summary

The CI pipeline runs integration tests via the existing integration-test job but never runs the 162 backend unit tests in tensormap-backend/tests/. This PR adds a parallel backend-unit-tests job.

Changes

• Renamed workflow from Integration Tests to Tests to reflect broader scope
• Added backend-unit-tests job that runs pytest tests/ -v --tb=short via uv
• Both jobs run in parallel on PRs to main

Validation

• All 162 backend unit tests pass locally on Python 3.12
• No changes to application code — CI workflow only
• Single commit, rebased on latest main

Files Changed

• .github/workflows/integration-tests.yml — added parallel backend unit test job