_ensure_postgres_schema_with_pool runs inside the drained pool context but
acquired its connection with a raw 'async with pool.connection()'. Caller
cancellation arriving while the connection's __aexit__ is blocked interrupts
that exit, so the connection is never returned to the pool and the pool's own
drained teardown closes with a checked-out connection — the backend-ownership
invariant documented in runtime/AGENTS.md (same class as the provider-context
fix that introduced drained_async_context).
Use drained_async_context for the schema-setup connection so the connection
returns to the pool before the pool context unwinds.
