Address backend implementation review findings

internal/backend/registry.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/jsonvalue"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
 )
 
 const (
@@ -75,7 +77,7 @@ func (r *Registry) GetBackend(id string) (domain.Backend, error) {
 	if !ok {
 		return domain.Backend{}, fmt.Errorf("%w: %q", ErrBackendNotFound, id)
 	}
-	extraParams, err := copyJSONMap(definition.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
 	if err != nil {
 		return domain.Backend{}, fmt.Errorf("copy backend %q: %w", id, err)
 	}
@@ -83,25 +85,6 @@ func (r *Registry) GetBackend(id string) (domain.Backend, error) {
 	return definition, nil
 }
 
-// IsReservedRequestField reports whether name is owned by the standard
-// OpenAI-compatible chat request rather than backend extra parameters.
-func IsReservedRequestField(name string) bool {
-	switch name {
-	case "model",
-		"session_id",
-		"messages",
-		"temperature",
-		"max_tokens",
-		"top_p",
-		"service_tier",
-		"reasoning_effort",
-		"response_format":
-		return true
-	default:
-		return false
-	}
-}
-
 func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 	definition.Endpoint = strings.TrimSpace(definition.Endpoint)
 	if err := validateEndpoint(definition.Endpoint); err != nil {
@@ -126,7 +109,7 @@ func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 		if key == "" {
 			return domain.Backend{}, fmt.Errorf("backend %q extra parameter key must not be empty", definition.ID)
 		}
-		if IsReservedRequestField(key) {
+		if llm.IsReservedOpenAIChatRequestField(key) {
 			return domain.Backend{}, fmt.Errorf(
 				"backend %q extra parameter %q collides with a reserved request field",
 				definition.ID,
@@ -135,7 +118,7 @@ func normalizeBackend(definition domain.Backend) (domain.Backend, error) {
 		}
 	}
 
-	extraParams, err := copyJSONMap(definition.ExtraParams)
+	extraParams, err := jsonvalue.CopyMap(definition.ExtraParams)
 	if err != nil {
 		return domain.Backend{}, fmt.Errorf("backend %q extra parameters: %w", definition.ID, err)
 	}