Address backend implementation review findings

internal/llm/openai_compatible_client.go

@@ -14,7 +14,6 @@ import (
 	"time"
 	"unicode/utf8"
 
-	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
 	"gitea.maximumdirect.net/eric/promptkit/internal/defaults"
 	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
 )
@@ -263,7 +262,7 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 		if key == "" {
 			return nil, errors.New("extra_params key must not be empty")
 		}
-		if backend.IsReservedRequestField(key) {
+		if IsReservedOpenAIChatRequestField(key) {
 			return nil, fmt.Errorf("extra_params key %q collides with reserved request field", key)
 		}
 		if _, err := json.Marshal(value); err != nil {
@@ -275,6 +274,25 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 	return out, nil
 }
 
+// IsReservedOpenAIChatRequestField reports whether name is owned by the
+// standard OpenAI-compatible chat request rather than extra parameters.
+func IsReservedOpenAIChatRequestField(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
+	}
+}
+
 type openAIChatRequestMessage struct {
 	Role    string `json:"role"`
 	Content any    `json:"content"`