fn: fix Result[T].FlatMap

For a Result[T], FlatMap should apply f when the result is Ok, and
propagate the error unchanged when it's Err. The original code returns r
on Ok and tries to use r.left when Err, which is wrong. This commit
fixes that.

Secondly, the group of FlatMap/AndThen and OrElse functions and methods
are now properly tested with new unit tests.

fixes #10401

fn/result.go

@@ -149,10 +149,10 @@ func FlattenResult[A any](r Result[Result[A]]) Result[A] {
 // success value if it exists.
 func (r Result[T]) FlatMap(f func(T) Result[T]) Result[T] {
 	if r.IsOk() {
-		return r
+		return f(r.left)
 	}
 
-	return f(r.left)
+	return r
 }
 
 // AndThen is an alias for FlatMap. This along with OrElse can be used to