feat(gateway): align body shape with real Claude Code CLI defaults

Three field-level alignments in normalizeClaudeOAuthRequestBody to
match real Claude Code CLI traffic byte-for-byte:

  1. temperature: previously deleted unconditionally; now passes
     through client value, defaults to 1 when absent (real CLI
     always sends temperature, default 1).

  2. max_tokens: defaults to 128000 when absent (real CLI default).

  3. context_management: when thinking.type is enabled/adaptive
     and the client did not provide context_management, inject
     {"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}
     to mirror real CLI behavior.

tool_choice removal is unchanged (Claude Code OAuth credentials
do not allow client-supplied tool_choice).

Tests updated:
  - gateway_body_order_test.go: temperature/max_tokens are now
    expected in output; tool_choice still removed.
  - gateway_prompt_test.go: system array is now 2 blocks
    (billing + cc prompt), assertions adjusted.
  - gateway_anthropic_apikey_passthrough_test.go: same 2-block
    assertion.

backend/internal/service/gateway_anthropic_apikey_passthrough_test.go

@@ -762,8 +762,14 @@ func TestGatewayService_AnthropicOAuth_ForwardPreservesBillingHeaderSystemBlock(
 			system := gjson.GetBytes(upstream.lastBody, "system")
 			require.True(t, system.Exists())
 			require.True(t, system.IsArray(), "system should be an array")
-			require.Equal(t, claudeCodeSystemPrompt, system.Array()[0].Get("text").String())
+			arr := system.Array()
-			require.Equal(t, "ephemeral", system.Array()[0].Get("cache_control.type").String())
+			require.Len(t, arr, 2, "system array should have billing block + cc prompt block")
+
+			require.Contains(t, arr[0].Get("text").String(), "x-anthropic-billing-header:")
+			require.Contains(t, arr[0].Get("text").String(), "cc_version=")
+
+			require.Equal(t, claudeCodeSystemPrompt, arr[1].Get("text").String())
+			require.Equal(t, "ephemeral", arr[1].Get("cache_control.type").String())
 
 			// 原始 system prompt 应迁移至 messages 中
 			messages := gjson.GetBytes(upstream.lastBody, "messages")

backend/internal/service/gateway_body_order_test.go

@@ -41,12 +41,13 @@ func TestNormalizeClaudeOAuthRequestBody_PreservesTopLevelFieldOrder(t *testing.
 	resultStr := string(result)
 
 	require.Equal(t, claude.NormalizeModelID("claude-3-5-sonnet-latest"), modelID)
-	assertJSONTokenOrder(t, resultStr, `"alpha"`, `"model"`, `"system"`, `"messages"`, `"omega"`, `"tools"`, `"metadata"`)
+	assertJSONTokenOrder(t, resultStr, `"alpha"`, `"model"`, `"temperature"`, `"system"`, `"messages"`, `"omega"`, `"tools"`, `"metadata"`, `"max_tokens"`)
-	require.NotContains(t, resultStr, `"temperature"`)
+	require.Contains(t, resultStr, `"temperature":0.2`)
 	require.NotContains(t, resultStr, `"tool_choice"`)
 	require.Contains(t, resultStr, `"system":"`+claudeCodeSystemPrompt+`"`)
 	require.Contains(t, resultStr, `"tools":[]`)
 	require.Contains(t, resultStr, `"metadata":{"user_id":"user-1"}`)
+	require.Contains(t, resultStr, `"max_tokens":128000`)
 }
 
 func TestInjectClaudeCodePrompt_PreservesFieldOrder(t *testing.T) {

backend/internal/service/gateway_prompt_test.go

@@ -378,16 +378,27 @@ func TestRewriteSystemForNonClaudeCode(t *testing.T) {
 			err := json.Unmarshal(result, &parsed)
 			require.NoError(t, err)
 
-			// system 应为 array 格式: [{type: "text", text: "...", cache_control: {type: "ephemeral"}}]
+			// system 应为 array 格式，对齐真实 Claude Code CLI 的 2-block 形态：
+			//   [0] billing attribution block (x-anthropic-billing-header: cc_version=...;)
+			//   [1] Claude Code prompt block (带 cache_control)
 			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")
+			require.Len(t, systemArr, 2, "system array should have exactly 2 blocks (billing + cc prompt)")
-			systemBlock, ok := systemArr[0].(map[string]any)
+
+			billingBlock, ok := systemArr[0].(map[string]any)
+			require.True(t, ok)
+			require.Equal(t, "text", billingBlock["type"])
+			require.Contains(t, billingBlock["text"], "x-anthropic-billing-header:")
+			require.Contains(t, billingBlock["text"], "cc_version=")
+			require.Contains(t, billingBlock["text"], "cc_entrypoint=cli")
+			require.Contains(t, billingBlock["text"], "cch=00000")
+
+			systemBlock, ok := systemArr[1].(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.True(t, ok, "cc prompt block should have cache_control")
 			require.Equal(t, "ephemeral", cc["type"])
 
 			// 检查 messages

backend/internal/service/gateway_service.go

@@ -1078,12 +1078,38 @@ func normalizeClaudeOAuthRequestBody(body []byte, modelID string, opts claudeOAu
 		}
 	}
 
-	if gjson.GetBytes(out, "temperature").Exists() {
+	// temperature：真实 Claude Code CLI 总是发送 temperature（默认 1，客户端可覆盖）。
-		if next, ok := deleteJSONPathBytes(out, "temperature"); ok {
+	// 之前的实现直接 delete 会导致 payload 缺字段，与真实 CLI 字节级不一致。
+	// 策略：客户端传了什么就透传；没传则补默认 1。
+	if !gjson.GetBytes(out, "temperature").Exists() {
+		if next, ok := setJSONValueBytes(out, "temperature", 1); ok {
 			out = next
 			modified = true
 		}
 	}
+
+	// max_tokens：真实 CLI 的默认值是 128000。缺失时补齐以对齐指纹。
+	if !gjson.GetBytes(out, "max_tokens").Exists() {
+		if next, ok := setJSONValueBytes(out, "max_tokens", 128000); ok {
+			out = next
+			modified = true
+		}
+	}
+
+	// context_management：thinking.type 为 enabled/adaptive 时，真实 CLI 会自动
+	// 附带 {"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}。
+	// 客户端显式传了就透传；否则按 CLI 行为补齐。
+	if !gjson.GetBytes(out, "context_management").Exists() {
+		thinkingType := gjson.GetBytes(out, "thinking.type").String()
+		if thinkingType == "enabled" || thinkingType == "adaptive" {
+			const cmDefault = `{"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}`
+			if next, ok := setJSONRawBytes(out, "context_management", []byte(cmDefault)); ok {
+				out = next
+				modified = true
+			}
+		}
+	}
+
 	if gjson.GetBytes(out, "tool_choice").Exists() {
 		if next, ok := deleteJSONPathBytes(out, "tool_choice"); ok {
 			out = next