Fix backfill UI active state for running task instances

[bobu-putheeckal]

Closes #64827.\n\nThis updates the private UI backfills endpoint to include the number of running task instances associated with each backfill and treats a backfill as active while any of those task instances are still running. The backfill banner now keeps rendering/refetching for that case, including backwards backfills where a newer run has completed while an older task instance is still running.\n\nTests:\n- uv run --no-dev --with pytest --with-editable ./devel-common --with-editable ./providers/git pytest airflow-core/tests/unit/api_fastapi/core_api/routes/ui/test_backfills.py -q\n- uv run --no-dev --with ruff ruff check airflow-core/src/airflow/api_fastapi/core_api/datamodels/backfills.py airflow-core/src/airflow/api_fastapi/core_api/routes/ui/backfills.py airflow-core/tests/unit/api_fastapi/core_api/routes/ui/test_backfills.py\n- python3 -m py_compile airflow-core/src/airflow/api_fastapi/core_api/datamodels/backfills.py airflow-core/src/airflow/api_fastapi/core_api/routes/ui/backfills.py airflow-core/tests/unit/api_fastapi/core_api/routes/ui/test_backfills.py\n- git diff --check

[kaxil]

PR template hasn't been followed on any of your PRs. As such I am closing all your PRs since I see little to no evidence of actual testing and mass PRs to Airflow & other repos.