fix: MapKeys returns empty slice for natively-typed maps

Get() preserves the concrete type of a map[string]T value loaded via
Set() or a provider like structs, but MapKeys() only type-asserted
map[string]any, so it silently reported no keys for anything else.
Read keys via reflection instead so any map with string keys works.

tests/koanf_test.go

@@ -1533,6 +1533,17 @@ func TestBoolsNativeSlice(t *testing.T) {
 	assert.Equal([]bool{true, false, true}, k.Bools("bools"))
 }
 
+func TestMapKeysNativeMap(t *testing.T) {
+	assert := assert.New(t)
+
+	k := koanf.New(delim)
+	assert.NoError(k.Load(confmap.Provider(map[string]any{
+		"ints": map[string]int64{"a": 1, "b": 2},
+	}, "."), nil))
+
+	assert.Equal([]string{"a", "b"}, k.MapKeys("ints"), "map keys mismatch for a natively-typed map")
+}
+
 // waitTimeout waits for the waitgroup for the specified max timeout.
 // Returns true if waiting timed out.
 func waitTimeout(wg *sync.WaitGroup, timeout time.Duration) bool {