perf: use load_only() in get_dag_runs eager loading to reduce data fetched per task instance (#62482)

* perf: use load_only() in eager_load_dag_run_for_validation to reduce data fetched

The get_dag_runs API endpoint was slow on large deployments because
eager_load_dag_run_for_validation() used selectinload on task_instances and
task_instances_histories without restricting which columns were fetched.
This caused SQLAlchemy to load all heavyweight columns (executor_config with
pickled data, hostname, rendered fields, etc.) for every task instance across
every DAG run in the result page — even though only dag_version_id is needed
to traverse the association proxy to DagVersion.

Add load_only(TaskInstance.dag_version_id) and
load_only(TaskInstanceHistory.dag_version_id) to the selectinload chains so
the SELECT for task instances fetches only the identity columns and the FK
needed to resolve the dag_version relationship, significantly reducing the
volume of data transferred from the database on busy deployments.

Fixes #62025

* Fix static checks

---------

Co-authored-by: pierrejeambrun <pierrejbrun@gmail.com>

Author: LakshmiSravyaVedantham

airflow-core/src/airflow/api_fastapi/common/db/dag_runs.py

@@ -41,13 +41,24 @@
 
 
 def eager_load_dag_run_for_validation() -> tuple[LoaderOption, ...]:
-    """Construct the eager loading options necessary for a DagRunResponse object."""
+    """
+    Construct the eager loading options necessary for a DagRunResponse object.
+
+    For the list endpoint (get_dag_runs), loading all task instance columns is
+    wasteful because we only need the dag_version_id FK to traverse to DagVersion.
+    Using load_only() on TaskInstance and TaskInstanceHistory restricts the SELECT
+    to just the identity columns and dag_version_id, avoiding large intermediate
+    result sets caused by loading heavyweight columns (executor_config, etc.) for
+    every task instance across every DAG run returned by the query.
+    """
     return (
         joinedload(DagRun.dag_model),
         selectinload(DagRun.task_instances)
+        .load_only(TaskInstance.dag_version_id)
         .joinedload(TaskInstance.dag_version)
         .joinedload(DagVersion.bundle),
         selectinload(DagRun.task_instances_histories)
+        .load_only(TaskInstanceHistory.dag_version_id)
         .joinedload(TaskInstanceHistory.dag_version)
         .joinedload(DagVersion.bundle),
         joinedload(DagRun.dag_run_note),