Fix dag parsing persistence when bundle cache is missing after processor restart

[GayathriSrividya]

closes #69249

What this PR changes

• make dag parsing persistence robust when the in-memory bundle version cache has no entry for a bundle
• use a safe lookup in DagFileProcessorManager.handle_parsing_result so persistence still proceeds with bundle_version=None
• add a regression test that verifies parsing results are persisted when the bundle version cache is missing

Why this fixes the issue

After a dag-processor restart, parsing can proceed before _bundle_versions is populated for a bundle. The previous direct index access raised KeyError, which short-circuited persistence and prevented normal DAG metadata updates. Falling back to None keeps persistence moving and allows stale DAG state to be refreshed instead of remaining stuck.

Tests

• /opt/homebrew/bin/ruff format airflow-core/src/airflow/dag_processing/manager.py airflow-core/tests/unit/dag_processing/test_manager.py
• /opt/homebrew/bin/ruff check --fix airflow-core/src/airflow/dag_processing/manager.py airflow-core/tests/unit/dag_processing/test_manager.py
• uv run --project airflow-core pytest airflow-core/tests/unit/dag_processing/test_manager.py -k "bundle_version_cache_is_missing or updates_stats_after_successful_persist or throttles_retry_when_first_persist_fails" -xvs (fails during collection in this environment with ModuleNotFoundError: airflow.providers.hashicorp before running tests)