refactor(serialization): generalize unmarshal/load functions with pluggable format handlers

Replace YAML-specific functions with generic ones accepting unmarshaler/marshaler
function parameters. This enables future support for JSON and other formats
while maintaining current YAML behavior.

- UnmarshalValidateYAML -> UnmarshalValidate(unmarshalFunc)
- UnmarshalValidateYAMLXSync -> UnmarshalValidateXSync(unmarshalFunc)
- SaveJSON -> SaveFile(marshalFunc)
- LoadJSONIfExist -> LoadFileIfExist(unmarshalFunc)
- Add UnmarshalValidateReader for reader-based decoding

Testing: all 12 staged test files updated to use new API

internal/jsonstore/jsonstore.go

@@ -82,7 +82,8 @@ func loadNS[T store](ns namespace) T {
 func save() error {
 	errs := gperr.NewBuilder("failed to save data stores")
 	for ns, store := range stores {
-		if err := serialization.SaveJSON(filepath.Join(storesPath, string(ns)+".json"), &store, 0o644); err != nil {
+		path := filepath.Join(storesPath, string(ns)+".json")
+		if err := serialization.SaveFile(path, &store, 0o644, sonic.Marshal); err != nil {
 			errs.Add(err)
 		}
 	}