fix(gateway): 修复 OpenAI→Anthropic 转换路径 system prompt 被静默丢弃的 bug

injectClaudeCodePrompt 和 systemIncludesClaudeCodePrompt 的 type switch
无法匹配 json.RawMessage 类型（Go typed nil 陷阱），导致 ForwardAsResponses
和 ForwardAsChatCompletions 路径中用户 system prompt 被替换为仅 Claude Code
banner。新增 normalizeSystemParam 将 json.RawMessage 转为标准 Go 类型。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/gateway_prompt_test.go

@@ -124,6 +124,27 @@ func TestSystemIncludesClaudeCodePrompt(t *testing.T) {
 			},
 			want: false,
 		},
+		// json.RawMessage cases (conversion path: ForwardAsResponses / ForwardAsChatCompletions)
+		{
+			name:   "json.RawMessage string with Claude Code prompt",
+			system: json.RawMessage(`"` + claudeCodeSystemPrompt + `"`),
+			want:   true,
+		},
+		{
+			name:   "json.RawMessage string without Claude Code prompt",
+			system: json.RawMessage(`"You are a helpful assistant"`),
+			want:   false,
+		},
+		{
+			name:   "json.RawMessage nil (empty)",
+			system: json.RawMessage(nil),
+			want:   false,
+		},
+		{
+			name:   "json.RawMessage empty string",
+			system: json.RawMessage(`""`),
+			want:   false,
+		},
 	}
 
 	for _, tt := range tests {
@@ -202,6 +223,29 @@ func TestInjectClaudeCodePrompt(t *testing.T) {
 			wantSystemLen: 1,
 			wantFirstText: claudeCodeSystemPrompt,
 		},
+		// json.RawMessage cases (conversion path: ForwardAsResponses / ForwardAsChatCompletions)
+		{
+			name:           "json.RawMessage string system",
+			body:           `{"model":"claude-3","system":"Custom prompt"}`,
+			system:         json.RawMessage(`"Custom prompt"`),
+			wantSystemLen:  2,
+			wantFirstText:  claudeCodeSystemPrompt,
+			wantSecondText: claudePrefix + "\n\nCustom prompt",
+		},
+		{
+			name:          "json.RawMessage nil system",
+			body:          `{"model":"claude-3"}`,
+			system:        json.RawMessage(nil),
+			wantSystemLen: 1,
+			wantFirstText: claudeCodeSystemPrompt,
+		},
+		{
+			name:          "json.RawMessage Claude Code prompt (should not duplicate)",
+			body:          `{"model":"claude-3","system":"` + claudeCodeSystemPrompt + `"}`,
+			system:        json.RawMessage(`"` + claudeCodeSystemPrompt + `"`),
+			wantSystemLen: 1,
+			wantFirstText: claudeCodeSystemPrompt,
+		},
 	}
 
 	for _, tt := range tests {

backend/internal/service/gateway_service.go

@@ -3749,9 +3749,28 @@ func isClaudeCodeRequest(ctx context.Context, c *gin.Context, parsed *ParsedRequ
 	return isClaudeCodeClient(c.GetHeader("User-Agent"), parsed.MetadataUserID)
 }
 
+// normalizeSystemParam 将 json.RawMessage 类型的 system 参数转为标准 Go 类型（string / []any / nil），
+// 避免 type switch 中 json.RawMessage（底层 []byte）无法匹配 case string / case []any / case nil 的问题。
+// 这是 Go 的 typed nil 陷阱：(json.RawMessage, nil) ≠ (nil, nil)。
+func normalizeSystemParam(system any) any {
+	raw, ok := system.(json.RawMessage)
+	if !ok {
+		return system
+	}
+	if len(raw) == 0 {
+		return nil
+	}
+	var parsed any
+	if err := json.Unmarshal(raw, &parsed); err != nil {
+		return nil
+	}
+	return parsed
+}
+
 // systemIncludesClaudeCodePrompt 检查 system 中是否已包含 Claude Code 提示词
 // 使用前缀匹配支持多种变体（标准版、Agent SDK 版等）
 func systemIncludesClaudeCodePrompt(system any) bool {
+	system = normalizeSystemParam(system)
 	switch v := system.(type) {
 	case string:
 		return hasClaudeCodePrefix(v)
@@ -3780,6 +3799,7 @@ func hasClaudeCodePrefix(text string) bool {
 // injectClaudeCodePrompt 在 system 开头注入 Claude Code 提示词
 // 处理 null、字符串、数组三种格式
 func injectClaudeCodePrompt(body []byte, system any) []byte {
+	system = normalizeSystemParam(system)
 	claudeCodeBlock, err := marshalAnthropicSystemTextBlock(claudeCodeSystemPrompt, true)
 	if err != nil {
 		logger.LegacyPrintf("service.gateway", "Warning: failed to build Claude Code prompt block: %v", err)