fix: Anthropic 流式编码器补全 message_start/message_delta 必填字段

跨协议流式转换时，Anthropic 客户端 Zod 校验因 SSE 事件缺少必填字段报错。
由 Anthropic encoder 层（而非 OpenAI decoder 层）负责补全协议默认值，保持权责分离。

- encodeMessageStart 补全 type/content/stop_reason/stop_sequence，usage nil 时输出零值
- encodeMessageDelta usage nil 时输出零值
- 更新相关测试覆盖新增行为

backend/internal/conversion/anthropic/stream_encoder.go

@@ -50,16 +50,24 @@ func (e *StreamEncoder) encodeMessageStart(event canonical.CanonicalStreamEvent)
 	}
 	if event.Message != nil {
 		msg := map[string]any{
-			"id":    event.Message.ID,
-			"model": event.Message.Model,
-			"role":  "assistant",
+			"id":            event.Message.ID,
+			"type":          "message",
+			"role":          "assistant",
+			"content":       []any{},
+			"model":         event.Message.Model,
+			"stop_reason":   nil,
+			"stop_sequence": nil,
 		}
 		if event.Message.Usage != nil {
-			usage := map[string]any{
+			msg["usage"] = map[string]any{
 				"input_tokens":  event.Message.Usage.InputTokens,
 				"output_tokens": event.Message.Usage.OutputTokens,
 			}
-			msg["usage"] = usage
+		} else {
+			msg["usage"] = map[string]any{
+				"input_tokens":  0,
+				"output_tokens": 0,
+			}
 		}
 		payload["message"] = msg
 	}
@@ -147,6 +155,10 @@ func (e *StreamEncoder) encodeMessageDelta(event canonical.CanonicalStreamEvent)
 		payload["usage"] = map[string]any{
 			"output_tokens": event.Usage.OutputTokens,
 		}
+	} else {
+		payload["usage"] = map[string]any{
+			"output_tokens": 0,
+		}
 	}
 	return e.marshalEvent("message_delta", payload)
 }

backend/internal/conversion/anthropic/stream_encoder_test.go

@@ -21,8 +21,55 @@ func TestStreamEncoder_MessageStart(t *testing.T) {
 	s := string(chunks[0])
 	assert.True(t, strings.HasPrefix(s, "event: message_start\n"))
 	assert.Contains(t, s, "data: ")
-	assert.Contains(t, s, "msg_1")
-	assert.Contains(t, s, "claude-3")
+
+	var payload map[string]any
+	lines := strings.Split(s, "\n")
+	for _, l := range lines {
+		if strings.HasPrefix(l, "data: ") {
+			require.NoError(t, json.Unmarshal([]byte(strings.TrimPrefix(l, "data: ")), &payload))
+			break
+		}
+	}
+
+	msg, ok := payload["message"].(map[string]any)
+	require.True(t, ok)
+	assert.Equal(t, "msg_1", msg["id"])
+	assert.Equal(t, "message", msg["type"])
+	assert.Equal(t, "assistant", msg["role"])
+	assert.Equal(t, []any{}, msg["content"])
+	assert.Equal(t, "claude-3", msg["model"])
+	assert.Nil(t, msg["stop_reason"])
+	assert.Nil(t, msg["stop_sequence"])
+
+	usage, okU := msg["usage"].(map[string]any)
+	require.True(t, okU)
+	assert.Equal(t, float64(0), usage["input_tokens"])
+	assert.Equal(t, float64(0), usage["output_tokens"])
+}
+
+func TestStreamEncoder_MessageStart_WithUsage(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewMessageStartEventWithUsage("msg_2", "gpt-4", &canonical.CanonicalUsage{InputTokens: 100, OutputTokens: 50})
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+
+	s := string(chunks[0])
+	var payload map[string]any
+	lines := strings.Split(s, "\n")
+	for _, l := range lines {
+		if strings.HasPrefix(l, "data: ") {
+			require.NoError(t, json.Unmarshal([]byte(strings.TrimPrefix(l, "data: ")), &payload))
+			break
+		}
+	}
+
+	msg, ok := payload["message"].(map[string]any)
+	require.True(t, ok)
+	usage, okU := msg["usage"].(map[string]any)
+	require.True(t, okU)
+	assert.Equal(t, float64(100), usage["input_tokens"])
+	assert.Equal(t, float64(50), usage["output_tokens"])
 }
 
 func TestStreamEncoder_ContentBlockDelta(t *testing.T) {
@@ -179,6 +226,10 @@ func TestStreamEncoder_MessageDelta_WithStopReason(t *testing.T) {
 	delta, okd := payload["delta"].(map[string]any)
 	require.True(t, okd)
 	assert.Equal(t, "end_turn", delta["stop_reason"])
+
+	usage, oku := payload["usage"].(map[string]any)
+	require.True(t, oku, "message_delta SHALL always include usage")
+	assert.Equal(t, float64(0), usage["output_tokens"])
 }
 
 func TestStreamEncoder_MessageDelta_WithUsage(t *testing.T) {

openspec/specs/protocol-adapter-anthropic/spec.md

@@ -239,6 +239,26 @@ Decoder 几乎 1:1 映射，维护最小状态机：
 - **WHEN** delta.type == "thinking_delta"
 - **THEN** SHALL 编码为 Anthropic thinking_delta
 
+#### Scenario: message_start 事件编码完整 message 字段
+
+- **WHEN** 编码 MessageStartEvent
+- **THEN** SHALL 输出 `message` 对象包含以下字段：
+  - `id`: 来自 event.Message.ID
+  - `type`: 固定值 `"message"`
+  - `role`: 固定值 `"assistant"`
+  - `content`: 固定值 `[]`（空数组）
+  - `model`: 来自 event.Message.Model
+  - `stop_reason`: 固定值 `null`
+  - `stop_sequence`: 固定值 `null`
+  - `usage`: 来自 event.Message.Usage；若 Usage 为 nil，SHALL 输出 `{"input_tokens": 0, "output_tokens": 0}`
+
+#### Scenario: message_delta 事件编码包含 usage 字段
+
+- **WHEN** 编码 MessageDeltaEvent
+- **THEN** SHALL 输出 `usage` 字段
+- **WHEN** event.Usage 为 nil
+- **THEN** SHALL 输出 `{"output_tokens": 0}`
+
 ### Requirement: Anthropic 错误编码
 
 系统 SHALL 实现 Anthropic 协议的错误编码。