Fix transaction release race

[simPod]

This fixes a transaction release race in PostgresHandleConnection.

The first commit adds a regression test that opens a transaction, commits it, and immediately opens another transaction. Before the fix, the previous transaction's close callback releases the connection lock asynchronously, so the second beginTransaction() can observe the connection as still busy.

Observed before the fix with assertions enabled:

assert($this->busy === null) in src/Internal/PostgresHandleConnection.php:85

Observed before the fix with assertions disabled, matching the production symptom:

Amp\Future\UnhandledFutureError: Unhandled future: Error: "Call to a member function complete() on null"
Caused by Error: Call to a member function complete() on null
src/Internal/PostgresHandleConnection.php:96
vendor/amphp/sql-common/src/SqlConnectionTransaction.php:100

The second commit makes beginTransaction() wait for any pending transaction release before reserving the connection again, matching the behavior already used by query, execute, prepare, notify, and listen.