This change fixes a situation where error handling was skipped during
`processCommit()` in case the `doCommit()` failed. The error handling
was set up via an `onErrorResume` operator that was nested inside a
`then(...)`, applied to an inner `Mono.empty()`. As a consequence,
it would never receive an error signal (effectively decoupling the
onErrorResume from the main chain).
This change simply moves the error handling back one level up. It also
simplifies the `doCommit` code a bit by getting rid of the steps that
artificially introduce a `Mono<Object>` return type, which is not really
needed.
A pre-existing test was missing the fact that the rollback didn't occur,
which is now fixed. Another dedicated test is introduced building upon
the `ReactiveTestTransactionManager` class.
Closes gh-30096
9b50c0d590