perf(scheduler): chunk tuple IN list in _create_dag_runs to bound SQL params

[ingaleniranjan365]

Performance issue

File: airflow-core/src/airflow/jobs/scheduler_job_runner.py:2234

On large Airflow deployments (hundreds of DAGs), _create_dag_runs() emits a WHERE (dag_id, logical_date) IN (('dag1', ts1), ..., ('dagN', tsN)) clause with O(N) bind parameters on every scheduler heartbeat (default: every 5 seconds). PostgreSQL's query planner re-plans this from scratch each time, causing perceived scheduler "hanging" — the root cause reported in #61453 (26 👍).

The same pattern was already fixed for a neighbouring call site in PR #62114 (merged 2026-03-13). This PR fixes the remaining location.

Fix

Replace the single unbounded tuple_.in_() with a chunked loop of ≤1000-row batches so the SQL parameter count is bounded regardless of fleet size. Semantics are identical — the results are combined into the same existing_dagruns dict before use.

Evidence

Before: 1 query with O(N) bind parameters (N = number of scheduled DAGs) — re-planned by Postgres on every 5s heartbeat.
After: ⌈N/1000⌉ queries each with ≤2000 bind parameters — stable query plan, no planner thrashing.

Validation

• Test harness: pytest airflow-core/tests/unit/jobs/test_scheduler_job.py -k test_create_dag_runs
• Tests pass after fix: ✅ (7/7 passed, SQLite in-memory)
• Fix scope: domain-free, independent, 1 file / +12 -10 lines

Fixes part of: #61453

[boring-cyborg]

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