Suspicious nested transaction behaviour code

### Expected Behavior

A `@Transactional(propagation = Transactional.Propagation.NESTED)` should create a new transaction, if there's an existing one, according to the documentation of NESTED:
> Execute within a nested transaction if a current transaction exists

### Actual Behaviour

The existing transaction seems to be reused. See the relevant code in [AbstractTransactionOperations](https://github.com/micronaut-projects/micronaut-data/blob/4.11.x/data-tx/src/main/java/io/micronaut/transaction/support/AbstractTransactionOperations.java#L469)

### Steps To Reproduce

1. checkout demo app from https://github.com/sdedic/micronaut-dbbugs1
2. checkout branch `nested-transaction`
3. place a breakpoint on [line in AbstractTransactionOperations](https://github.com/micronaut-projects/micronaut-data/blob/4.11.x/data-tx/src/main/java/io/micronaut/transaction/support/AbstractTransactionOperations.java#L469)
4. execute the application with parameter `-3`

No nested transaction is opened. Transaction closes with the outermost one.

### Environment Information

- Ubuntu 22.04
- JDK 21

### Example Application

_No response_

### Version

4.7.6

Start with data-tx/src/main/java/io/micronaut/transaction/support/AbstractTransactionOperations.java at line 469. Reproduce the report using the linked demo app on its nested-transaction branch, running it with parameter -3, and inspect whether the existing transaction is reused. Done means NESTED propagation opens a nested transaction when an outer transaction exists and closes at the nested boundary.

Written by the indexing model from the issue text.