Ensure reactive transaction rollback on commit error

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

spring-r2dbc/src/test/java/org/springframework/r2dbc/connection/R2dbcTransactionManagerUnitTests.java

@@ -270,6 +270,7 @@ class R2dbcTransactionManagerUnitTests {
 		verify(connectionMock).beginTransaction(any(io.r2dbc.spi.TransactionDefinition.class));
 		verify(connectionMock).createStatement("foo");
 		verify(connectionMock).commitTransaction();
+		verify(connectionMock).rollbackTransaction();
 		verify(connectionMock).close();
 		verifyNoMoreInteractions(connectionMock);
 	}