fix: 非Claude Code客户端system prompt迁移至messages以绕过第三方应用检测

Anthropic近期引入基于system参数内容的第三方应用检测机制，原有的前置追加
Claude Code提示词策略无法通过检测（后续内容仍为非Claude Code格式触发429）。

新策略：对非Claude Code客户端的OAuth/SetupToken账号请求，将system字段
完整替换为Claude Code标识提示词，原始system内容作为user/assistant消息对
注入messages开头，模型仍接收完整指令。

仅影响/v1/messages路径，chat_completions和responses路径保持原有逻辑不变。
真正的Claude Code客户端请求完全不受影响（原样透传）。

backend/internal/service/gateway_prompt_test.go

@@ -278,3 +278,141 @@ func TestInjectClaudeCodePrompt(t *testing.T) {
 		})
 	}
 }
+
+func TestRewriteSystemForNonClaudeCode(t *testing.T) {
+	tests := []struct {
+		name             string
+		body             string
+		system           any
+		wantSystemStr    string // system 应为纯字符串
+		wantMessagesLen  int    // messages 数组长度
+		wantFirstMsgRole string // 第一条消息的 role
+		wantFirstMsgText string // 第一条消息的 content[0].text
+		wantAckMsgText   string // 第二条消息的 content[0].text
+	}{
+		{
+			name:            "nil system - no messages injected",
+			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:          nil,
+			wantSystemStr:   claudeCodeSystemPrompt,
+			wantMessagesLen: 1, // 原始 1 条消息，不注入
+		},
+		{
+			name:            "empty string system - no messages injected",
+			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:          "",
+			wantSystemStr:   claudeCodeSystemPrompt,
+			wantMessagesLen: 1,
+		},
+		{
+			name:             "custom string system - migrated to messages",
+			body:             `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:           "You are a personal assistant running inside OpenClaw.",
+			wantSystemStr:    claudeCodeSystemPrompt,
+			wantMessagesLen:  3, // instruction + ack + original
+			wantFirstMsgRole: "user",
+			wantFirstMsgText: "[System Instructions]\nYou are a personal assistant running inside OpenClaw.",
+			wantAckMsgText:   "Understood. I will follow these instructions.",
+		},
+		{
+			name:            "system equals Claude Code prompt - no messages injected",
+			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:          claudeCodeSystemPrompt,
+			wantSystemStr:   claudeCodeSystemPrompt,
+			wantMessagesLen: 1,
+		},
+		{
+			name: "array system with custom blocks - text joined and migrated",
+			body: `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system: []any{
+				map[string]any{"type": "text", "text": "First instruction"},
+				map[string]any{"type": "text", "text": "Second instruction"},
+			},
+			wantSystemStr:    claudeCodeSystemPrompt,
+			wantMessagesLen:  3,
+			wantFirstMsgRole: "user",
+			wantFirstMsgText: "[System Instructions]\nFirst instruction\n\nSecond instruction",
+			wantAckMsgText:   "Understood. I will follow these instructions.",
+		},
+		{
+			name:            "empty array system - no messages injected",
+			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:          []any{},
+			wantSystemStr:   claudeCodeSystemPrompt,
+			wantMessagesLen: 1,
+		},
+		{
+			name:             "json.RawMessage string system",
+			body:             `{"model":"claude-3","system":"Custom prompt","messages":[{"role":"user","content":"hello"}]}`,
+			system:           json.RawMessage(`"Custom prompt"`),
+			wantSystemStr:    claudeCodeSystemPrompt,
+			wantMessagesLen:  3,
+			wantFirstMsgRole: "user",
+			wantFirstMsgText: "[System Instructions]\nCustom prompt",
+			wantAckMsgText:   "Understood. I will follow these instructions.",
+		},
+		{
+			name:            "json.RawMessage nil system",
+			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
+			system:          json.RawMessage(nil),
+			wantSystemStr:   claudeCodeSystemPrompt,
+			wantMessagesLen: 1,
+		},
+		{
+			name:             "multiple original messages preserved",
+			body:             `{"model":"claude-3","messages":[{"role":"user","content":"msg1"},{"role":"assistant","content":"resp1"},{"role":"user","content":"msg2"}]}`,
+			system:           "Be helpful",
+			wantSystemStr:    claudeCodeSystemPrompt,
+			wantMessagesLen:  5, // 2 injected + 3 original
+			wantFirstMsgRole: "user",
+			wantFirstMsgText: "[System Instructions]\nBe helpful",
+			wantAckMsgText:   "Understood. I will follow these instructions.",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := rewriteSystemForNonClaudeCode([]byte(tt.body), tt.system)
+
+			var parsed map[string]any
+			err := json.Unmarshal(result, &parsed)
+			require.NoError(t, err)
+
+			// system 应为纯字符串
+			systemVal, ok := parsed["system"].(string)
+			require.True(t, ok, "system should be a string, got %T", parsed["system"])
+			require.Equal(t, tt.wantSystemStr, systemVal)
+
+			// 检查 messages
+			messages, ok := parsed["messages"].([]any)
+			require.True(t, ok, "messages should be an array")
+			require.Len(t, messages, tt.wantMessagesLen)
+
+			if tt.wantFirstMsgRole != "" && len(messages) >= 2 {
+				// 检查注入的 instruction 消息
+				firstMsg, ok := messages[0].(map[string]any)
+				require.True(t, ok)
+				require.Equal(t, tt.wantFirstMsgRole, firstMsg["role"])
+
+				firstContent, ok := firstMsg["content"].([]any)
+				require.True(t, ok)
+				require.Len(t, firstContent, 1)
+				firstBlock, ok := firstContent[0].(map[string]any)
+				require.True(t, ok)
+				require.Equal(t, tt.wantFirstMsgText, firstBlock["text"])
+
+				// 检查注入的 ack 消息
+				ackMsg, ok := messages[1].(map[string]any)
+				require.True(t, ok)
+				require.Equal(t, "assistant", ackMsg["role"])
+
+				ackContent, ok := ackMsg["content"].([]any)
+				require.True(t, ok)
+				require.Len(t, ackContent, 1)
+				ackBlock, ok := ackContent[0].(map[string]any)
+				require.True(t, ok)
+				require.Equal(t, tt.wantAckMsgText, ackBlock["text"])
+			}
+		})
+	}
+}