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/canonical/types_test.go

@@ -0,0 +1,114 @@
+package canonical
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetSystemString(t *testing.T) {
+	tests := []struct {
+		name    string
+		system  any
+		want    string
+	}{
+		{"string", "hello", "hello"},
+		{"nil", nil, ""},
+		{"empty string", "", ""},
+		{"system blocks", []SystemBlock{{Text: "part1"}, {Text: "part2"}}, "part1\n\npart2"},
+		{"single block", []SystemBlock{{Text: "only"}}, "only"},
+		{"other type", 123, "123"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := &CanonicalRequest{System: tt.system}
+			assert.Equal(t, tt.want, req.GetSystemString())
+		})
+	}
+}
+
+func TestSetSystemString(t *testing.T) {
+	req := &CanonicalRequest{}
+
+	req.SetSystemString("hello")
+	assert.Equal(t, "hello", req.System)
+
+	req.SetSystemString("")
+	assert.Nil(t, req.System)
+}
+
+func TestNewTextBlock(t *testing.T) {
+	b := NewTextBlock("hello")
+	assert.Equal(t, "text", b.Type)
+	assert.Equal(t, "hello", b.Text)
+}
+
+func TestNewToolUseBlock(t *testing.T) {
+	input := json.RawMessage(`{"key":"val"}`)
+	b := NewToolUseBlock("id-1", "tool_name", input)
+	assert.Equal(t, "tool_use", b.Type)
+	assert.Equal(t, "id-1", b.ID)
+	assert.Equal(t, "tool_name", b.Name)
+	assert.Equal(t, input, b.Input)
+}
+
+func TestNewToolResultBlock(t *testing.T) {
+	b := NewToolResultBlock("tool-1", "result", false)
+	assert.Equal(t, "tool_result", b.Type)
+	assert.Equal(t, "tool-1", b.ToolUseID)
+	assert.NotNil(t, b.IsError)
+	assert.False(t, *b.IsError)
+}
+
+func TestNewThinkingBlock(t *testing.T) {
+	b := NewThinkingBlock("thought")
+	assert.Equal(t, "thinking", b.Type)
+	assert.Equal(t, "thought", b.Thinking)
+}
+
+func TestNewToolChoice(t *testing.T) {
+	assert.Equal(t, &ToolChoice{Type: "auto"}, NewToolChoiceAuto())
+	assert.Equal(t, &ToolChoice{Type: "none"}, NewToolChoiceNone())
+	assert.Equal(t, &ToolChoice{Type: "any"}, NewToolChoiceAny())
+	assert.Equal(t, &ToolChoice{Type: "tool", Name: "fn"}, NewToolChoiceNamed("fn"))
+}
+
+func TestCanonicalRequest_RoundTrip(t *testing.T) {
+	req := &CanonicalRequest{
+		Model:    "gpt-4",
+		System:   "system prompt",
+		Messages: []CanonicalMessage{{Role: RoleUser, Content: []ContentBlock{NewTextBlock("hi")}}},
+		Stream:   true,
+	}
+
+	data, err := json.Marshal(req)
+	require.NoError(t, err)
+
+	var decoded CanonicalRequest
+	require.NoError(t, json.Unmarshal(data, &decoded))
+	assert.Equal(t, "gpt-4", decoded.Model)
+	assert.Equal(t, "system prompt", decoded.System)
+	assert.True(t, decoded.Stream)
+}
+
+func TestCanonicalResponse_RoundTrip(t *testing.T) {
+	sr := StopReasonEndTurn
+	resp := &CanonicalResponse{
+		ID:      "resp-1",
+		Model:   "gpt-4",
+		Content: []ContentBlock{NewTextBlock("hello")},
+		StopReason: &sr,
+		Usage:   CanonicalUsage{InputTokens: 10, OutputTokens: 5},
+	}
+
+	data, err := json.Marshal(resp)
+	require.NoError(t, err)
+
+	var decoded CanonicalResponse
+	require.NoError(t, json.Unmarshal(data, &decoded))
+	assert.Equal(t, "resp-1", decoded.ID)
+	assert.Equal(t, StopReasonEndTurn, *decoded.StopReason)
+}