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`).

internal/registry/codex_client_models.go

@@ -0,0 +1,11 @@
+package registry
+
+import _ "embed"
+
+//go:embed models/codex_client_models.json
+var codexClientModelsJSON []byte
+
+// GetCodexClientModelsJSON returns the embedded Codex client model catalog.
+func GetCodexClientModelsJSON() []byte {
+	return append([]byte(nil), codexClientModelsJSON...)
+}