fix(node-postgres): release pool client when BEGIN rejects in transaction()

[allocsys]

Fixes #6023

Problem

NodePgSession.transaction() checks a client out of a pg.Pool, then runs BEGIN before entering the try/finally that calls release(). If BEGIN rejects (e.g. the connection drops right after checkout), control skips both the catch (which would ROLLBACK) and the finally (which releases the client) — the checked-out client is never returned to the pool. Repeated failures can exhaust it.

Fix

Move BEGIN inside the existing try/finally and track whether it actually succeeded via a began flag, so ROLLBACK is only attempted once a transaction has genuinely started, while release() still runs on every exit path:

let began = false;
try {
  await tx.execute(sql`begin${config ? sql` ${tx.getTransactionConfigSQL(config)}` : undefined}`);
  began = true;
  const result = await transaction(tx);
  await tx.execute(sql`commit`);
  return result;
} catch (error) {
  if (began) await tx.execute(sql`rollback`);
  throw error;
} finally {
  if (isPool) (session.client as PoolClient).release();
}

Verification

Ran the minimal repro from the issue (a FailingPool/client stub whose query always rejects, simulating a dropped connection on BEGIN) directly against the src in both states:

• Unpatched main: released: 0 — leak reproduces
• This branch: released: 1 — client released, no leak

Note: the currently-published drizzle-orm@0.45.2 (npm latest) already contains an equivalent guard, so current stable users aren't affected — this brings main in line with that existing fix. Left more detail on this in the issue thread.

Scope note

Nested transactions (NodePgTransaction.transaction(), the savepoint path) have a similar "no guard around the initial statement" shape, but there's no pool client to leak there since it reuses the existing session — left out of this PR to keep it minimal/single-purpose. Happy to follow up separately if maintainers want that covered too.