Add multi-team lookup to Lockbox backend

[PrithviBadiga]

Adds multi-team lookup support to LockboxSecretBackend.

Updates:

• look up team-scoped secrets first when team_name is provided
• fall back to the global secret when no team-scoped secret exists
• avoid resolving team-scoped identifiers as global secrets when team_name is not provided
• document the Lockbox team-scoped naming convention

Lookup behavior:

• team-scoped: {prefix}/{team_name}/{secret_id}
• global fallback: {prefix}/{secret_id}

Verification:

• AIRFLOW_HOME=$(mktemp -d) PYTHONPATH=/Users/prith/Desktop/Codex/airflow-65682/airflow-core/src:/Users/prith/Desktop/Codex/airflow-65682/providers/yandex/src /Users/prith/Desktop/Codex/airflow/.venv/bin/python -m pytest /Users/prith/Desktop/Codex/airflow-65689-yandex/providers/yandex/tests/unit/yandex/secrets/test_lockbox.py
• /Users/prith/Desktop/Codex/airflow/.venv/bin/python -m ruff check /Users/prith/Desktop/Codex/airflow-65689-yandex/providers/yandex/src/airflow/providers/yandex/secrets/lockbox.py /Users/prith/Desktop/Codex/airflow-65689-yandex/providers/yandex/tests/unit/yandex/secrets/test_lockbox.py
• /Users/prith/Desktop/Codex/airflow/.venv/bin/python -m ruff format --check /Users/prith/Desktop/Codex/airflow-65689-yandex/providers/yandex/src/airflow/providers/yandex/secrets/lockbox.py /Users/prith/Desktop/Codex/airflow-65689-yandex/providers/yandex/tests/unit/yandex/secrets/test_lockbox.py

Part of: #65682