Fix backfill API session handling for SQLite

[bobu-putheeckal]

Summary

• pass the request-scoped API session into _create_backfill when creating backfills
• keep existing CLI/model behavior by letting _create_backfill create its own session when one is not supplied
• add a regression test that fails if the API path opens a separate backfill model session

Fixes #66726

Testing

• uv run --no-dev --package apache-airflow-core --with 'apache-airflow-devel-common[basic]' ruff check airflow-core/src/airflow/models/backfill.py airflow-core/src/airflow/api_fastapi/core_api/routes/public/backfills.py airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_backfills.py
• PYTHONPATH="airflow-core/src:airflow-core/tests:devel-common/src:providers/git/src:providers/standard/src:providers/fab/src:shared/configuration/src:shared/dagnode/src:shared/listeners/src:shared/logging/src:shared/module_loading/src:shared/observability/src:shared/plugins_manager/src:shared/providers_discovery/src:shared/secrets_backend/src:shared/secrets_masker/src:shared/serialization/src:shared/state/src:shared/template_rendering/src:shared/timezones/src" uv run --python 3.12 --no-dev --package apache-airflow-core --with-editable ./devel-common --with 'apache-airflow-devel-common[basic]' --with-editable ./providers/git --with-editable ./providers/standard --with-editable ./providers/fab python -m pytest airflow-core/tests/unit/api_fastapi/core_api/routes/public/test_backfills.py::TestCreateBackfill -q

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[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.