refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

- 新增 ConversionEngine 核心引擎，支持 OpenAI 和 Anthropic 协议转换
- 添加 stream decoder/encoder 实现
- 更新 provider client 支持新引擎
- 补充单元测试和集成测试
- 更新 specs 文档

backend/internal/conversion/openai/stream_decoder_test.go

@@ -353,3 +353,120 @@ func TestStreamDecoder_MultipleChunks_Text(t *testing.T) {
 	}
 	assert.Equal(t, []string{"你好", "世界"}, deltas)
 }
+
+func TestStreamDecoder_UTF8Truncation(t *testing.T) {
+	d := NewStreamDecoder()
+
+	chunk := map[string]any{
+		"id":     "chatcmpl-utf8",
+		"model":  "gpt-4",
+		"choices": []any{
+			map[string]any{
+				"index": 0,
+				"delta": map[string]any{"content": "你"},
+			},
+		},
+	}
+	data, _ := json.Marshal(chunk)
+	sseData := []byte("data: " + string(data) + "\n\n")
+
+	mid := len(sseData) - 5
+	part1 := sseData[:mid]
+	part2 := sseData[mid:]
+
+	events1 := d.ProcessChunk(part1)
+	for _, e := range events1 {
+		if e.Type == canonical.EventContentBlockDelta && e.Delta != nil {
+			assert.Equal(t, "你", e.Delta.Text)
+		}
+	}
+
+	events2 := d.ProcessChunk(part2)
+	_ = events2
+}
+
+func TestStreamDecoder_ToolCallSubsequentDelta(t *testing.T) {
+	d := NewStreamDecoder()
+
+	idx := 0
+	chunk1 := map[string]any{
+		"id":     "chatcmpl-tc",
+		"model":  "gpt-4",
+		"choices": []any{
+			map[string]any{
+				"index": 0,
+				"delta": map[string]any{
+					"tool_calls": []any{
+						map[string]any{
+							"index": &idx,
+							"id":    "call_1",
+							"type":  "function",
+							"function": map[string]any{
+								"name":      "get_weather",
+								"arguments": "",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+	chunk2 := map[string]any{
+		"id":     "chatcmpl-tc",
+		"model":  "gpt-4",
+		"choices": []any{
+			map[string]any{
+				"index": 0,
+				"delta": map[string]any{
+					"tool_calls": []any{
+						map[string]any{
+							"index": &idx,
+							"function": map[string]any{
+								"arguments": "{\"city\":\"Beijing\"}",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	events1 := d.ProcessChunk(makeChunkSSE(chunk1))
+	require.NotEmpty(t, events1)
+
+	events2 := d.ProcessChunk(makeChunkSSE(chunk2))
+	require.NotEmpty(t, events2)
+
+	foundInputJSON := false
+	for _, e := range events2 {
+		if e.Type == canonical.EventContentBlockDelta && e.Delta != nil && e.Delta.Type == "input_json_delta" {
+			foundInputJSON = true
+			assert.Equal(t, "{\"city\":\"Beijing\"}", e.Delta.PartialJSON)
+		}
+	}
+	assert.True(t, foundInputJSON, "subsequent tool call delta should emit input_json_delta")
+}
+
+func TestStreamDecoder_InvalidJSON(t *testing.T) {
+	d := NewStreamDecoder()
+
+	raw := []byte("data: {invalid json}\n\n")
+	events := d.ProcessChunk(raw)
+	assert.Nil(t, events)
+}
+
+func TestStreamDecoder_NonDataLines(t *testing.T) {
+	d := NewStreamDecoder()
+
+	raw := []byte(": comment line\ndata: {\"id\":\"1\",\"choices\":[{\"delta\":{\"content\":\"hi\"}}]}\n\n")
+	events := d.ProcessChunk(raw)
+	require.NotEmpty(t, events)
+	found := false
+	for _, e := range events {
+		if e.Type == canonical.EventContentBlockDelta && e.Delta != nil {
+			found = true
+			assert.Equal(t, "hi", e.Delta.Text)
+		}
+	}
+	assert.True(t, found)
+}