Add Gemini-to-Gemini request normalization and passthrough support

- Introduced a `ConvertGeminiRequestToGemini` function to normalize Gemini v1beta requests by ensuring valid or default roles.
- Added passthrough response handlers for both streamed and non-streamed Gemini responses.
- Registered translators for Gemini-to-Gemini traffic in the initialization process.
- Updated `gemini-cli` request normalization to align with the new Gemini translator logic.

internal/translator/gemini/gemini/gemini_gemini_response.go

@@ -0,0 +1,15 @@
+package gemini
+
+import (
+	"context"
+)
+
+// PassthroughGeminiResponseStream forwards Gemini responses unchanged.
+func PassthroughGeminiResponseStream(_ context.Context, _ string, rawJSON []byte, _ *any) []string {
+	return []string{string(rawJSON)}
+}
+
+// PassthroughGeminiResponseNonStream forwards Gemini responses unchanged.
+func PassthroughGeminiResponseNonStream(_ context.Context, _ string, rawJSON []byte, _ *any) string {
+	return string(rawJSON)
+}