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.

examples/translator/main.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/translator"
+	_ "github.com/router-for-me/CLIProxyAPI/v6/sdk/translator/builtin"
+)
+
+func main() {
+	rawRequest := []byte(`{"messages":[{"content":[{"text":"Hello! Gemini","type":"text"}],"role":"user"}],"model":"gemini-2.5-pro","stream":false}`)
+	fmt.Println("Has gemini->openai response translator:", translator.HasResponseTransformerByFormatName(
+		translator.FormatGemini,
+		translator.FormatOpenAI,
+	))
+
+	translatedRequest := translator.TranslateRequestByFormatName(
+		translator.FormatOpenAI,
+		translator.FormatGemini,
+		"gemini-2.5-pro",
+		rawRequest,
+		false,
+	)
+
+	fmt.Printf("Translated request to Gemini format:\n%s\n\n", translatedRequest)
+
+	claudeResponse := []byte(`{"candidates":[{"content":{"role":"model","parts":[{"thought":true,"text":"Okay, here's what's going through my mind. I need to schedule a meeting"},{"thoughtSignature":"","functionCall":{"name":"schedule_meeting","args":{"topic":"Q3 planning","attendees":["Bob","Alice"],"time":"10:00","date":"2025-03-27"}}}]},"finishReason":"STOP","avgLogprobs":-0.50018133435930523}],"usageMetadata":{"promptTokenCount":117,"candidatesTokenCount":28,"totalTokenCount":474,"trafficType":"PROVISIONED_THROUGHPUT","promptTokensDetails":[{"modality":"TEXT","tokenCount":117}],"candidatesTokensDetails":[{"modality":"TEXT","tokenCount":28}],"thoughtsTokenCount":329},"modelVersion":"gemini-2.5-pro","createTime":"2025-08-15T04:12:55.249090Z","responseId":"x7OeaIKaD6CU48APvNXDyA4"}`)
+
+	convertedResponse := translator.TranslateNonStreamByFormatName(
+		context.Background(),
+		translator.FormatGemini,
+		translator.FormatOpenAI,
+		"gemini-2.5-pro",
+		rawRequest,
+		translatedRequest,
+		claudeResponse,
+		nil,
+	)
+
+	fmt.Printf("Converted response for OpenAI clients:\n%s\n", convertedResponse)
+}