[v4] fix flatten map util function (#211)

* add the fixed version of FlattenMap() and corresponding tests
* replaced the old FlattenMap() implementation with a new one

go.mod

@@ -12,6 +12,7 @@ require (
 	github.com/patrickmn/go-cache v2.1.0+incompatible
 	github.com/silas/dag v0.0.0-20220518035006-a7e85ada93c5
 	github.com/spf13/cast v1.7.1
+	github.com/stretchr/testify v1.10.0
 	go.uber.org/atomic v1.11.0
 	go.uber.org/automaxprocs v1.6.0
 	go.unistack.org/micro-proto/v4 v4.1.0
@@ -26,7 +27,6 @@ require (
 	github.com/pbnjay/memory v0.0.0-20210728143218-7b4eea64cf58 // indirect
 	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/rogpeppe/go-internal v1.13.1 // indirect
-	github.com/stretchr/testify v1.10.0 // indirect
 	golang.org/x/net v0.34.0 // indirect
 	golang.org/x/sys v0.29.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20241216192217-9240e9c98484 // indirect