Mutiny.Session#currentTransaction not updated when transaction is opened by TransactionalContextPool

[lucamolteni]

In Quarkus PR #51063 (Support @transactional for Hibernate Reactive, addressing #47698), we implemented
automatic transaction management by opening transactions in TransactionalContextPool.getConnection():

  @Override
  public Future<SqlConnection> getConnection() {
      if (!shouldOpenTransaction()) {
          return delegate.getConnection();
      } else {
          return delegate.getConnection()
              .compose(connection -> {
                  return connection.begin().map(t -> {
                      Transaction transaction = connection.transaction();
                      Vertx.currentContext().putLocal(CURRENT_TRANSACTION_KEY, transaction);
                      return new TransactionalContextConnection(connection);
                  });
              });
      }
  }

While the transaction is started and stored in the Vert.x context, Hibernate Reactive's
Mutiny.Session#currentTransaction field is never updated. This means that calling
session.currentTransaction() returns null even though a transaction is active.

As noted by @yrodiere:

If we open transactions "under the hood", we will need Hibernate Reactive to correctly detect that and
expose Mutiny.Session#currentTransaction accordingly.

Expected Behavior:

There should be a way in Hibernate Reactive to fix state so that wen a transaction is opened by TransactionalContextPool, Mutiny.Session#currentTransaction() should
return the active transaction.

Actual Behavior:

Mutiny.Session#currentTransaction() returns null because the session's internal currentTransaction
field is not updated when the transaction is opened at the connection pool level.

Impact:

• Code that relies on session.currentTransaction() to check if a transaction is active will not work
  correctly
• Transaction state is not visible through the Hibernate Reactive API