feat(translator): add built-in translator registry and helpers

- Introduced `builtin` package exposing a default registry and pipeline for built-in translators.
- Added format constants for common schemas (e.g., OpenAI, Gemini, Codex).
- Implemented helper functions for schema translation using format name strings.
- Provided example usage for integration with translator helpers.

sdk/translator/builtin/builtin.go

@@ -0,0 +1,18 @@
+// Package builtin exposes the built-in translator registrations for SDK users.
+package builtin
+
+import (
+	sdktranslator "github.com/router-for-me/CLIProxyAPI/v6/sdk/translator"
+
+	_ "github.com/router-for-me/CLIProxyAPI/v6/internal/translator"
+)
+
+// Registry exposes the default registry populated with all built-in translators.
+func Registry() *sdktranslator.Registry {
+	return sdktranslator.Default()
+}
+
+// Pipeline returns a pipeline that already contains the built-in translators.
+func Pipeline() *sdktranslator.Pipeline {
+	return sdktranslator.NewPipeline(sdktranslator.Default())
+}