Polish Behavior api

The current one may be nested; we can make use of a chained call.
eg:
https://smallrye.io/smallrye-mutiny/latest/

```scala
request.ifNoItem().after(ofMillis(100))
.failWith(() -> new TooSlowException("💥"))
.onFailure(IOException.class).recoverWithItem(fail -> "📦")
.subscribe().with(
item -> log("👍 " + item),
err -> log(err.getMessage())
);
```

No Pekko file, test, or entry point is named in the issue. Start by reviewing the linked SmallRye Mutiny example and the existing Pekko behavior API, then clarify the intended chaining design and affected API surface with maintainers. Done should mean an agreed API shape with corresponding implementation and tests.

Written by the indexing model from the issue text.