feat: add database transaction state check

[memleakd]

Description

This PR proposes adding inTransaction() to database connections so code can check whether the connection is currently inside an active CodeIgniter-managed transaction.

The method provides a small public API for transaction state without exposing transaction depth or requiring userland code to rely on internal connection properties. This is useful for reusable services, libraries, and framework-adjacent integrations that need to adapt when they are called from inside an existing transaction.

For example, a package may need to decide whether to perform work immediately, join caller-managed transactional behavior, or defer side effects through transaction callbacks.

Behavior

• Returns true while the connection is inside a CodeIgniter-managed transaction.
• Returns false when no CodeIgniter-managed transaction is active.
• Returns false when transactions are disabled.
• Reflects nested transaction state through the existing transaction depth behavior.
• Returns false inside afterCommit() and afterRollback() callbacks because the transaction has already reached its terminal state.

This is intentionally a boolean state check only. It does not expose the transaction depth, and it reflects transactions started through CodeIgniter’s transaction APIs rather than raw SQL or driver-specific transaction handling.

This adds inTransaction() to ConnectionInterface, so custom implementations will need to add the new method. The changelog lists this under interface changes.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide