style: fix linting issues in store module (#15724)

## Description

- Return an exported type rather than an unexported type, from an exported function
- Assign error values to _
- Add reason to nolint directive
- Remove ineffectual assignment
- Remove checked cast to same type as value

---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] ~~provided a link to the relevant issue or specification~~
- [ ] ~~reviewed "Files changed" and left comments if necessary~~
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed all author checklist items have been addressed
- [ ] confirmed that this PR does not change production code

store/cachekv/internal/mergeiterator.go

@@ -24,7 +24,7 @@ type cacheMergeIterator struct {
 
 var _ types.Iterator = (*cacheMergeIterator)(nil)
 
-func NewCacheMergeIterator(parent, cache types.Iterator, ascending bool) *cacheMergeIterator { //nolint:revive
+func NewCacheMergeIterator(parent, cache types.Iterator, ascending bool) types.Iterator {
 	iter := &cacheMergeIterator{
 		parent:    parent,
 		cache:     cache,

store/iavl/tree_test.go

@@ -14,8 +14,8 @@ func TestImmutableTreePanics(t *testing.T) {
 	it := &immutableTree{immTree}
 	require.Panics(t, func() { it.Set([]byte{}, []byte{}) })
 	require.Panics(t, func() { it.Remove([]byte{}) })
-	require.Panics(t, func() { it.SaveVersion() })           //nolint:errcheck
-	require.Panics(t, func() { it.DeleteVersion(int64(1)) }) //nolint:errcheck
+	require.Panics(t, func() { _, _, _ = it.SaveVersion() })
+	require.Panics(t, func() { _ = it.DeleteVersion(int64(1)) })
 
 	imm, err := it.GetImmutable(1)
 	require.Error(t, err)

store/internal/conv/string_test.go

@@ -26,7 +26,7 @@ func (s *StringSuite) TestUnsafeStrToBytes() {
 		b := unsafeConvertStr()
 		runtime.GC()
 		<-time.NewTimer(2 * time.Millisecond).C
-		b2 := append(b, 'd') 
+		b2 := append(b, 'd')
 		s.Equal("abc", string(b))
 		s.Equal("abcd", string(b2))
 	}

store/mem/store.go

@@ -27,7 +27,7 @@ func NewStore() *Store {
 	return NewStoreWithDB(dbm.NewMemDB())
 }
 
-func NewStoreWithDB(db *dbm.MemDB) *Store { //nolint: interfacer
+func NewStoreWithDB(db *dbm.MemDB) *Store { //nolint: interfacer // Concrete return type is fine here.
 	return &Store{Store: dbadapter.Store{DB: db}}
 }
 

store/rootmulti/store.go

@@ -827,7 +827,6 @@ func (rs *Store) Snapshot(height uint64, protoWriter protoio.Writer) error {
 				node, err := exporter.Next()
 				if err == iavltree.ErrorExportDone {
 					rs.logger.Debug("snapshot Done", "store", store.name, "nodeCount", nodeCount)
-					nodeCount = 0
 					break
 				} else if err != nil {
 					return err

store/types/store_test.go

@@ -233,8 +233,6 @@ func TestNewTransientStoreKeys(t *testing.T) {
 func TestNewInfiniteGasMeter(t *testing.T) {
 	gm := NewInfiniteGasMeter()
 	require.NotNil(t, gm)
-	_, ok := gm.(GasMeter) //nolint:gosimple
-	require.True(t, ok)
 }
 
 func TestStoreTypes(t *testing.T) {