Add immutable backend registry foundation

internal/llm/openai_compatible_client.go

@@ -14,6 +14,7 @@ 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"
 )
@@ -262,7 +263,7 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 		if key == "" {
 			return nil, errors.New("extra_params key must not be empty")
 		}
-		if _, reserved := reservedOpenAIChatRequestFields[key]; reserved {
+		if backend.IsReservedRequestField(key) {
 			return nil, fmt.Errorf("extra_params key %q collides with reserved request field", key)
 		}
 		if _, err := json.Marshal(value); err != nil {
@@ -274,18 +275,6 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 	return out, nil
 }
 
-var reservedOpenAIChatRequestFields = map[string]struct{}{
-	"model":            {},
-	"session_id":       {},
-	"messages":         {},
-	"temperature":      {},
-	"max_tokens":       {},
-	"top_p":            {},
-	"service_tier":     {},
-	"reasoning_effort": {},
-	"response_format":  {},
-}
-
 type openAIChatRequestMessage struct {
 	Role    string `json:"role"`
 	Content any    `json:"content"`