fix: 修复非CC客户端OAuth伪装被Anthropic检测为第三方应用的问题

commit f3aa54b 的 rewriteSystemForNonClaudeCode 未能通过 Anthropic 第三方检测，
根因是两个关键信号与真实 Claude Code 不一致：

1. anthropic-beta 头缺少 claude-code-20250219：伪装路径主动将该 beta
   加入 drop set 并移除，但 Anthropic 依赖此 beta 识别 Claude Code 请求。
   修复：非 haiku 模型的伪装请求强制包含 claude-code beta。

2. system 字段使用 string 格式而非 array+cache_control：真实 Claude Code
   始终以 [{type,text,cache_control:{type:"ephemeral"}}] 发送 system，
   string 格式成为第三方检测信号。
   修复：rewriteSystemForNonClaudeCode 改为注入 array 格式。

附带调整：stripSystemCacheControl 按 system 是否被重写动态决定，
重写时保留 CC prompt 的 cache_control，未重写时（haiku/已含CC前缀）
保持原有剥离行为。

backend/internal/service/gateway_prompt_test.go

@@ -284,7 +284,7 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 		name             string
 		body             string
 		system           any
-		wantSystemStr    string // system 应为纯字符串
+		wantSystemText   string // system array 第一个 block 的 text
 		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
@@ -294,21 +294,21 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			name:            "nil system - no messages injected",
 			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
 			system:          nil,
-			wantSystemStr:   claudeCodeSystemPrompt,
+			wantSystemText:  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,
+			wantSystemText:  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,
+			wantSystemText:   claudeCodeSystemPrompt,
 			wantMessagesLen:  3, // instruction + ack + original
 			wantFirstMsgRole: "user",
 			wantFirstMsgText: "[System Instructions]\nYou are a personal assistant running inside OpenClaw.",
@@ -318,7 +318,7 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			name:            "system equals Claude Code prompt - no messages injected",
 			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
 			system:          claudeCodeSystemPrompt,
-			wantSystemStr:   claudeCodeSystemPrompt,
+			wantSystemText:  claudeCodeSystemPrompt,
 			wantMessagesLen: 1,
 		},
 		{
@@ -328,7 +328,7 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 				map[string]any{"type": "text", "text": "First instruction"},
 				map[string]any{"type": "text", "text": "Second instruction"},
 			},
-			wantSystemStr:    claudeCodeSystemPrompt,
+			wantSystemText:   claudeCodeSystemPrompt,
 			wantMessagesLen:  3,
 			wantFirstMsgRole: "user",
 			wantFirstMsgText: "[System Instructions]\nFirst instruction\n\nSecond instruction",
@@ -338,14 +338,14 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			name:            "empty array system - no messages injected",
 			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
 			system:          []any{},
-			wantSystemStr:   claudeCodeSystemPrompt,
+			wantSystemText:  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,
+			wantSystemText:   claudeCodeSystemPrompt,
 			wantMessagesLen:  3,
 			wantFirstMsgRole: "user",
 			wantFirstMsgText: "[System Instructions]\nCustom prompt",
@@ -355,14 +355,14 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			name:            "json.RawMessage nil system",
 			body:            `{"model":"claude-3","messages":[{"role":"user","content":"hello"}]}`,
 			system:          json.RawMessage(nil),
-			wantSystemStr:   claudeCodeSystemPrompt,
+			wantSystemText:  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,
+			wantSystemText:   claudeCodeSystemPrompt,
 			wantMessagesLen:  5, // 2 injected + 3 original
 			wantFirstMsgRole: "user",
 			wantFirstMsgText: "[System Instructions]\nBe helpful",
@@ -378,10 +378,17 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			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)
+			// system 应为 array 格式: [{type: "text", text: "...", cache_control: {type: "ephemeral"}}]
+			systemArr, ok := parsed["system"].([]any)
+			require.True(t, ok, "system should be an array, got %T", parsed["system"])
+			require.Len(t, systemArr, 1, "system array should have exactly 1 block")
+			systemBlock, ok := systemArr[0].(map[string]any)
+			require.True(t, ok)
+			require.Equal(t, "text", systemBlock["type"])
+			require.Equal(t, tt.wantSystemText, systemBlock["text"])
+			cc, ok := systemBlock["cache_control"].(map[string]any)
+			require.True(t, ok, "system block should have cache_control")
+			require.Equal(t, "ephemeral", cc["type"])
 
 			// 检查 messages
 			messages, ok := parsed["messages"].([]any)