Centralize D&D candidate JSON codecs

internal/modules/dnd/codec/npcs/codec.go

@@ -1,15 +1,13 @@
 package npcs
 
 import (
-	"bytes"
 	"embed"
-	"encoding/json"
 	"fmt"
-	"io"
 	"strings"
 
 	"gitea.maximumdirect.net/eric/notarius/internal/framework/contracts"
 	"gitea.maximumdirect.net/eric/notarius/internal/modules/dnd"
+	"gitea.maximumdirect.net/eric/notarius/internal/modules/dnd/codec/candidatejson"
 	"gitea.maximumdirect.net/eric/notarius/internal/modules/dnd/npcs/identity"
 )
 
@@ -60,11 +58,7 @@ func (c *Codec) Encode(value dnd.NPCList) ([]byte, error) {
 // EncodeCandidate provides the durable representation before semantic
 // validators have approved a value.
 func (c *Codec) EncodeCandidate(value dnd.NPCList) ([]byte, error) {
-	content, err := json.Marshal(value)
-	if err != nil {
-		return nil, fmt.Errorf("encode dnd npc list: %w", err)
-	}
-	return content, nil
+	return candidatejson.EncodeCandidate("dnd npc list", value)
 }
 
 func (c *Codec) Decode(content []byte) (dnd.NPCList, error) {
@@ -81,17 +75,7 @@ func (c *Codec) Decode(content []byte) (dnd.NPCList, error) {
 // DecodeCandidate reads one strict durable JSON value before semantic
 // validators have approved it.
 func (c *Codec) DecodeCandidate(content []byte) (dnd.NPCList, error) {
-	decoder := json.NewDecoder(bytes.NewReader(content))
-	decoder.DisallowUnknownFields()
-	var value dnd.NPCList
-	if err := decoder.Decode(&value); err != nil {
-		return dnd.NPCList{}, fmt.Errorf("decode dnd npc list: %w", err)
-	}
-	var trailing any
-	if err := decoder.Decode(&trailing); err != io.EOF {
-		return dnd.NPCList{}, fmt.Errorf("decode dnd npc list: multiple JSON values")
-	}
-	return value, nil
+	return candidatejson.DecodeCandidate[dnd.NPCList]("dnd npc list", content)
 }
 
 func validate(value dnd.NPCList) error {