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/init.go

@@ -0,0 +1,21 @@
+package gemini
+
+import (
+	. "github.com/luispater/CLIProxyAPI/internal/constant"
+	"github.com/luispater/CLIProxyAPI/internal/interfaces"
+	"github.com/luispater/CLIProxyAPI/internal/translator/translator"
+)
+
+// Register a no-op response translator and a request normalizer for Gemini→Gemini.
+// The request converter ensures missing or invalid roles are normalized to valid values.
+func init() {
+	translator.Register(
+		GEMINI,
+		GEMINI,
+		ConvertGeminiRequestToGemini,
+		interfaces.TranslateResponse{
+			Stream:    PassthroughGeminiResponseStream,
+			NonStream: PassthroughGeminiResponseNonStream,
+		},
+	)
+}