refactor(api): move Codex client model handling to registry package

- Relocated Codex client model JSON and related logic from `openai` package to `registry` for better modularity.
- Updated references to use `registry.GetCodexClientModelsJSON()` in loading logic.
- Extended test cases to cover additional field removals (`upgrade`, `availability_nux`).

sdk/api/handlers/openai/openai_handlers.go

@@ -8,7 +8,6 @@ package openai
 
 import (
 	"context"
-	_ "embed"
 	"encoding/json"
 	"fmt"
 	"net/http"
@@ -30,9 +29,6 @@ type OpenAIAPIHandler struct {
 	*handlers.BaseAPIHandler
 }
 
-//go:embed codex_client_models.json
-var codexClientModelsJSON []byte
-
 // NewOpenAIAPIHandler creates a new OpenAI API handlers instance.
 // It takes an BaseAPIHandler instance as input and returns an OpenAIAPIHandler.
 //