Add terminationGracePeriodSeconds support for PgBouncer in Helm chart

[antruigon]

What

Adds pgbouncer.terminationGracePeriodSeconds to the Helm chart, rendered on the PgBouncer Deployment's pod spec. Defaults to 120 to match the drain window of the chart's default preStop hook.

Why

The chart ships a default PgBouncer preStop hook that drains client connections for up to 120 seconds:

containerLifecycleHooks:
  preStop:
    exec:
      # Allow existing queries clients to complete within 120 seconds
      command: ["/bin/sh", "-c", "killall -INT pgbouncer && sleep 120"]

…but the Deployment never sets terminationGracePeriodSeconds, so the Kubernetes default of 30s SIGKILLs the pod mid-drain. On any node drain or eviction (cluster upgrades, autoscaler scale-down, AMI rolls) in-flight client connections are cut instead of drained — workers see psycopg2.OperationalError: server closed the connection unexpectedly, and tasks that were mid-query fail. We hit this in production during a routine EKS node-group AMI roll: one evicted PgBouncer replica cut ~880 in-flight connections and failed 21 zero-retry DAG runs.

Every other long-running component in the chart already exposes this knob (scheduler, workers.celery, triggerer, dagProcessor, statsd, redis, otelCollector); PgBouncer was the only one missing it, and it is arguably the component that needs it most since it holds everyone's DB connections.

Default choice

120, matching the shipped preStop. Note this is an upper bound, not a fixed wait: PgBouncer receives SIGINT immediately, stops accepting new connections, and exits as soon as the last client connection is released — so pods still terminate quickly under low load. Users who prefer the old behaviour can set it back to 30.

Changes

• chart/values.yaml — new pgbouncer.terminationGracePeriodSeconds: 120 next to the preStop hook it pairs with
• chart/values.schema.json — schema entry (integer, default 120, Kubernetes docs section)
• chart/templates/pgbouncer/pgbouncer-deployment.yaml — render the field on the pod spec
• chart/tests/helm_tests/other/test_pgbouncer.py — parametrized test for default + override (mirrors test_statsd.py)

All 86 tests in test_pgbouncer.py + test_pdb_pgbouncer.py pass locally.

[boring-cyborg]

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