fix: encapsulation violation in BasePreparedQuery class

[michalsn]

Description
This PR fixes an encapsulation violation where BasePreparedQuery was directly accessing the protected property BaseConnection::$transStatus, which goes against PHP’s visibility rules.

To resolve this, I introduced a new internal method, BaseConnection::handleTransStatus(), which provides controlled access for modifying the transaction status when a query fails during an active transaction.

The issue was originally spotted in the forum: https://forum.codeigniter.com/showthread.php?tid=93029

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide