See gh-22891

Includes combined imports of assertion methods and related polishing.

Closes #22889

Closes gh-22888

Aligned with invocation.reactive and transaction.reactive packages.

Closes gh-22887

Closes gh-22837

Closes gh-22870

We now use Flux.usingWhen() instead materialize/dematerialize operators
to reuse Reactor's resource closure.

Until usingWhen() accepts a BiFunction to consume error signals,
we need to map error signals outside of usingWhen which requires
re-wrapping of the ReactiveTransaction object.

doCleanupAfterCompletion gets now subscribed to when processing
cleanup after a new transaction.

Closes gh-22877

Closes gh-22869

Such a static unmodifiable TransactionDefinition with defaults can be used for getTransaction(null) calls, now also possible for getReactiveTransaction. Furthermore, it can be used for a simple TransactionalOperator.create(ReactiveTransactionManager) method without an internal dependency on the transaction.support package.

See gh-22646

Closes gh-22874

Closes gh-22867

Closes gh-22358

Introduces TransactionManager marker interface for PlatformTransactionManager as well as ReactiveTransactionManager, allowing for a common configuration type in TransactionAspectSupport and TransactionManagementConfigurer.

Closes gh-22590

Introduces TransactionExecution base interface for TransactionStatus as well as ReactiveTransaction. Renames getTransaction method to getReactiveTransaction, allowing for combined implementations of PlatformTransactionManager and ReactiveTransactionManager.

See gh-22646

Includes general revision of reactive transaction sources.

See gh-22646

This commit adds SPI interfaces to support reactive transactions
through spring-tx with optional dependencies to Project Reactor and
supportive implementations for TransactionalOperator and
AbstractReactiveTransactionManager.

Closes gh-22862

Prior to this when SpEL compiled an expression using the numeric
operators <, >, <= or >= then it would not create code that handled
nulls. Nulls can occur if a boxed numeric operand is used prior
to compilation, then it is nulled out. SpEL now creates null
handling bytecode.

Closes gh-22358

Closes gh-22803

Closes gh-22851

Closes gh-22853