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

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/internal/conversion/openai/stream_encoder_test.go

@@ -0,0 +1,172 @@
+package openai
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"nex/backend/internal/conversion/canonical"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStreamEncoder_MessageStart(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewMessageStartEvent("chatcmpl-1", "gpt-4")
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+
+	s := string(chunks[0])
+	assert.True(t, strings.HasPrefix(s, "data: "))
+	assert.Contains(t, s, "chatcmpl-1")
+	assert.Contains(t, s, "chat.completion.chunk")
+
+	var payload map[string]any
+	data := strings.TrimPrefix(s, "data: ")
+	data = strings.TrimRight(data, "\n")
+	require.NoError(t, json.Unmarshal([]byte(data), &payload))
+	choices := payload["choices"].([]any)
+	delta := choices[0].(map[string]any)["delta"].(map[string]any)
+	assert.Equal(t, "assistant", delta["role"])
+}
+
+func TestStreamEncoder_TextDelta(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{Type: "text_delta", Text: "你好"})
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "你好")
+}
+
+func TestStreamEncoder_MessageStop(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewMessageStopEvent()
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+	assert.Equal(t, "data: [DONE]\n\n", string(chunks[0]))
+}
+
+func TestStreamEncoder_Buffering(t *testing.T) {
+	e := NewStreamEncoder()
+
+	// ContentBlockStart 应被缓冲，不输出
+	startEvent := canonical.NewContentBlockStartEvent(0, canonical.StreamContentBlock{Type: "text", Text: ""})
+	chunks := e.EncodeEvent(startEvent)
+	assert.Nil(t, chunks)
+	assert.NotNil(t, e.bufferedStart)
+
+	// 第一个 delta 触发输出（清空缓冲）
+	deltaEvent := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{Type: "text_delta", Text: "hello"})
+	chunks = e.EncodeEvent(deltaEvent)
+	require.NotEmpty(t, chunks)
+	assert.Nil(t, e.bufferedStart)
+}
+
+func TestStreamEncoder_ContentBlockStop_ReturnsNil(t *testing.T) {
+	e := NewStreamEncoder()
+	idx := 0
+	event := canonical.CanonicalStreamEvent{
+		Type:  canonical.EventContentBlockStop,
+		Index: &idx,
+	}
+	chunks := e.EncodeEvent(event)
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_Ping_ReturnsNil(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewPingEvent()
+	chunks := e.EncodeEvent(event)
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_Error_ReturnsNil(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewErrorEvent("test_error", "测试错误")
+	chunks := e.EncodeEvent(event)
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_Flush_ReturnsNil(t *testing.T) {
+	e := NewStreamEncoder()
+	chunks := e.Flush()
+	assert.Nil(t, chunks)
+}
+
+func TestStreamEncoder_ThinkingDelta(t *testing.T) {
+	e := NewStreamEncoder()
+	event := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:     string(canonical.DeltaTypeThinking),
+		Thinking: "思考内容",
+	})
+
+	chunks := e.EncodeEvent(event)
+	require.Len(t, chunks, 1)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "reasoning_content")
+	assert.Contains(t, s, "思考内容")
+}
+
+func TestStreamEncoder_InputJSONDelta(t *testing.T) {
+	e := NewStreamEncoder()
+
+	e.EncodeEvent(canonical.NewContentBlockStartEvent(0, canonical.StreamContentBlock{
+		Type: "tool_use",
+		ID:   "call_1",
+		Name: "get_weather",
+	}))
+
+	event := canonical.NewContentBlockDeltaEvent(0, canonical.StreamDelta{
+		Type:        string(canonical.DeltaTypeInputJSON),
+		PartialJSON: "{\"city\":\"北京\"}",
+	})
+
+	chunks := e.EncodeEvent(event)
+	require.NotEmpty(t, chunks)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "tool_calls")
+	assert.Contains(t, s, "北京")
+}
+
+func TestStreamEncoder_MessageDelta_WithStopReason(t *testing.T) {
+	e := NewStreamEncoder()
+	sr := canonical.StopReasonEndTurn
+	event := canonical.CanonicalStreamEvent{
+		Type:       canonical.EventMessageDelta,
+		StopReason: &sr,
+	}
+
+	chunks := e.EncodeEvent(event)
+	require.NotEmpty(t, chunks)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "finish_reason")
+	assert.Contains(t, s, "stop")
+}
+
+func TestStreamEncoder_MessageDelta_WithUsage(t *testing.T) {
+	e := NewStreamEncoder()
+	usage := canonical.CanonicalUsage{
+		InputTokens:  100,
+		OutputTokens: 50,
+	}
+	event := canonical.CanonicalStreamEvent{
+		Type:  canonical.EventMessageDelta,
+		Usage: &usage,
+	}
+
+	chunks := e.EncodeEvent(event)
+	require.NotEmpty(t, chunks)
+
+	s := string(chunks[0])
+	assert.Contains(t, s, "usage")
+	assert.Contains(t, s, "prompt_tokens")
+}