fix: parse Gemini native request format in ParseGatewayRequest for correct session hash generation

ParseGatewayRequest only parsed Anthropic format (system/messages),
ignoring Gemini native format (systemInstruction/contents). This caused
GenerateSessionHash to produce identical hashes for all Gemini sessions.

Add protocol parameter to ParseGatewayRequest to branch between
Anthropic and Gemini parsing. Update GenerateSessionHash message
traversal to extract text from both formats.

backend/internal/service/gateway_request_test.go

@@ -4,12 +4,13 @@ import (
 	"encoding/json"
 	"testing"
 
+	"github.com/Wei-Shaw/sub2api/internal/domain"
 	"github.com/stretchr/testify/require"
 )
 
 func TestParseGatewayRequest(t *testing.T) {
 	body := []byte(`{"model":"claude-3-7-sonnet","stream":true,"metadata":{"user_id":"session_123e4567-e89b-12d3-a456-426614174000"},"system":[{"type":"text","text":"hello","cache_control":{"type":"ephemeral"}}],"messages":[{"content":"hi"}]}`)
-	parsed, err := ParseGatewayRequest(body)
+	parsed, err := ParseGatewayRequest(body, "")
 	require.NoError(t, err)
 	require.Equal(t, "claude-3-7-sonnet", parsed.Model)
 	require.True(t, parsed.Stream)
@@ -22,7 +23,7 @@ func TestParseGatewayRequest(t *testing.T) {
 
 func TestParseGatewayRequest_ThinkingEnabled(t *testing.T) {
 	body := []byte(`{"model":"claude-sonnet-4-5","thinking":{"type":"enabled"},"messages":[{"content":"hi"}]}`)
-	parsed, err := ParseGatewayRequest(body)
+	parsed, err := ParseGatewayRequest(body, "")
 	require.NoError(t, err)
 	require.Equal(t, "claude-sonnet-4-5", parsed.Model)
 	require.True(t, parsed.ThinkingEnabled)
@@ -30,21 +31,21 @@ func TestParseGatewayRequest_ThinkingEnabled(t *testing.T) {
 
 func TestParseGatewayRequest_MaxTokens(t *testing.T) {
 	body := []byte(`{"model":"claude-haiku-4-5","max_tokens":1}`)
-	parsed, err := ParseGatewayRequest(body)
+	parsed, err := ParseGatewayRequest(body, "")
 	require.NoError(t, err)
 	require.Equal(t, 1, parsed.MaxTokens)
 }
 
 func TestParseGatewayRequest_MaxTokensNonIntegralIgnored(t *testing.T) {
 	body := []byte(`{"model":"claude-haiku-4-5","max_tokens":1.5}`)
-	parsed, err := ParseGatewayRequest(body)
+	parsed, err := ParseGatewayRequest(body, "")
 	require.NoError(t, err)
 	require.Equal(t, 0, parsed.MaxTokens)
 }
 
 func TestParseGatewayRequest_SystemNull(t *testing.T) {
 	body := []byte(`{"model":"claude-3","system":null}`)
-	parsed, err := ParseGatewayRequest(body)
+	parsed, err := ParseGatewayRequest(body, "")
 	require.NoError(t, err)
 	// 显式传入 system:null 也应视为“字段已存在”，避免默认 system 被注入。
 	require.True(t, parsed.HasSystem)
@@ -53,16 +54,111 @@ func TestParseGatewayRequest_SystemNull(t *testing.T) {
 
 func TestParseGatewayRequest_InvalidModelType(t *testing.T) {
 	body := []byte(`{"model":123}`)
-	_, err := ParseGatewayRequest(body)
+	_, err := ParseGatewayRequest(body, "")
 	require.Error(t, err)
 }
 
 func TestParseGatewayRequest_InvalidStreamType(t *testing.T) {
 	body := []byte(`{"stream":"true"}`)
-	_, err := ParseGatewayRequest(body)
+	_, err := ParseGatewayRequest(body, "")
 	require.Error(t, err)
 }
 
+// ============ Gemini 原生格式解析测试 ============
+
+func TestParseGatewayRequest_GeminiContents(t *testing.T) {
+	body := []byte(`{
+		"contents": [
+			{"role": "user", "parts": [{"text": "Hello"}]},
+			{"role": "model", "parts": [{"text": "Hi there"}]},
+			{"role": "user", "parts": [{"text": "How are you?"}]}
+		]
+	}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.Len(t, parsed.Messages, 3, "should parse contents as Messages")
+	require.False(t, parsed.HasSystem, "Gemini format should not set HasSystem")
+	require.Nil(t, parsed.System, "no systemInstruction means nil System")
+}
+
+func TestParseGatewayRequest_GeminiSystemInstruction(t *testing.T) {
+	body := []byte(`{
+		"systemInstruction": {
+			"parts": [{"text": "You are a helpful assistant."}]
+		},
+		"contents": [
+			{"role": "user", "parts": [{"text": "Hello"}]}
+		]
+	}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.NotNil(t, parsed.System, "should parse systemInstruction.parts as System")
+	parts, ok := parsed.System.([]any)
+	require.True(t, ok)
+	require.Len(t, parts, 1)
+	partMap, ok := parts[0].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "You are a helpful assistant.", partMap["text"])
+	require.Len(t, parsed.Messages, 1)
+}
+
+func TestParseGatewayRequest_GeminiWithModel(t *testing.T) {
+	body := []byte(`{
+		"model": "gemini-2.5-pro",
+		"contents": [{"role": "user", "parts": [{"text": "test"}]}]
+	}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.Equal(t, "gemini-2.5-pro", parsed.Model)
+	require.Len(t, parsed.Messages, 1)
+}
+
+func TestParseGatewayRequest_GeminiIgnoresAnthropicFields(t *testing.T) {
+	// Gemini 格式下 system/messages 字段应被忽略
+	body := []byte(`{
+		"system": "should be ignored",
+		"messages": [{"role": "user", "content": "ignored"}],
+		"contents": [{"role": "user", "parts": [{"text": "real content"}]}]
+	}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.False(t, parsed.HasSystem, "Gemini protocol should not parse Anthropic system field")
+	require.Nil(t, parsed.System, "no systemInstruction = nil System")
+	require.Len(t, parsed.Messages, 1, "should use contents, not messages")
+}
+
+func TestParseGatewayRequest_GeminiEmptyContents(t *testing.T) {
+	body := []byte(`{"contents": []}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.Empty(t, parsed.Messages)
+}
+
+func TestParseGatewayRequest_GeminiNoContents(t *testing.T) {
+	body := []byte(`{"model": "gemini-2.5-flash"}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformGemini)
+	require.NoError(t, err)
+	require.Nil(t, parsed.Messages)
+	require.Equal(t, "gemini-2.5-flash", parsed.Model)
+}
+
+func TestParseGatewayRequest_AnthropicIgnoresGeminiFields(t *testing.T) {
+	// Anthropic 格式下 contents/systemInstruction 字段应被忽略
+	body := []byte(`{
+		"system": "real system",
+		"messages": [{"role": "user", "content": "real content"}],
+		"contents": [{"role": "user", "parts": [{"text": "ignored"}]}],
+		"systemInstruction": {"parts": [{"text": "ignored"}]}
+	}`)
+	parsed, err := ParseGatewayRequest(body, domain.PlatformAnthropic)
+	require.NoError(t, err)
+	require.True(t, parsed.HasSystem)
+	require.Equal(t, "real system", parsed.System)
+	require.Len(t, parsed.Messages, 1)
+	msg := parsed.Messages[0].(map[string]any)
+	require.Equal(t, "real content", msg["content"])
+}
+
 func TestFilterThinkingBlocks(t *testing.T) {
 	containsThinkingBlock := func(body []byte) bool {
 		var req map[string]any