Add tests for `MissingMergedAnnotation`

See gh-21697

Delete `AnnotationAttributeExtractor` which is package private and
no longer used.

Closes gh-22753

Commit c0b52d09 introduced support for
throwing an exception if a @RequestMapping handler method in a Spring
MVC controller was mapped to an empty path. This had negative side
effects for applications that intentionally mapped to an empty path,
potentially alongside a mapping to an explicit path for the same
handler method.

This commit addresses this by logging a warning (instead of throwing an
exception) if a @RequestMapping method is mapped only to empty paths.

This commit also introduces the same support for WebFlux-based
@RequestMapping handler methods.

Closes gh-22543

See gh-22543

Includes bringing registerBean constructor-vararg variants up to GenericApplicationContext, making AnnotationConfigApplicationContext a straightforward subclass with a single template method to override.

See gh-22457

Closes gh-22742

Closes gh-22705

See gh-22705

Normally failed requests fail the test but they're suppressed for some
reason (e.g. in async callback) then verify should still correctly
report the failures.

Closes gh-21799

This reverts commit 07e9f802.

This reverts commit 86fb7362.

This reverts commit 65365d91.

This reverts commit 13543f5e.

Flow is a Kotlin Coroutines related cold asynchronous
stream of the data, that emits from zero to N (where N
can be unbounded) values and completes normally or with
an exception.

It is conceptually the Coroutines equivalent of Flux with
an extension oriented API design, easy custom operator
capabilities and some suspending methods.

This commit leverages Flow <-> Flux interoperability
to support Flow on controller handler method parameters
or return values, and also adds Flow based extensions to
WebFlux.fn. It allows to reach a point when we can consider
Spring Framework officially supports Coroutines even if some
additional work remains to be done like adding
interoperability between Reactor and Coroutines contexts.

Flow is currently an experimental API that is expected to
become final before Spring Framework 5.2 GA.

Close gh-19975

Closes gh-21118

Prior to this commit, handler methods in Spring MVC controllers were
not required to provide explicit path mappings via @RequestMapping (or
any of its specializations such as @GetMapping). Such handler methods
were effectively mapped to all paths. Consequently, developers may have
unwittingly mapped all requests to a single handler method.

This commit addresses this by enforcing that @RequestMapping methods
are mapped to an explicit path. Note, however, that this is enforced
after type-level and method-level @RequestMapping information has been
merged.

Developers wishing to map to all paths should now add an explicit path
mapping to "/**" or "**".

Closes gh-22543

Closes gh-22735

See gh-18490

See gh-18490

WebFlux.fn RouterFunctionDsl#invoke and CoRouterFunctionDsl#invoke
were wrongly exposed on public API and have never been designed to
be used by end users, but rather only invoked from router { } or
coRouter { } builders.

To fix that, avoiding this method being accessible from the DSL and
for the sake of consistency with WebMvc.fn RouterFunctionDsl,
Spring Framework 5.2 turns public fun invoke() method to
an internal fun build() one.

As a consequence RouterFunctionDsl and CoRouterFunctionDsl are not
open anymore, they are expected to be extended via Kotlin
extensions if needed.

Closes gh-22736

Closes gh-22697

See gh-19975

See gh-18490