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/stream_test.go

@@ -0,0 +1,130 @@
+package conversion
+
+import (
+	"testing"
+
+	"nex/backend/internal/conversion/canonical"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPassthroughStreamConverter_ProcessChunk(t *testing.T) {
+	converter := NewPassthroughStreamConverter()
+	data := []byte("hello world")
+	result := converter.ProcessChunk(data)
+	assert.Len(t, result, 1)
+	assert.Equal(t, data, result[0])
+}
+
+func TestPassthroughStreamConverter_Flush(t *testing.T) {
+	converter := NewPassthroughStreamConverter()
+	result := converter.Flush()
+	assert.Nil(t, result)
+}
+
+// mockStreamDecoder 模拟流式解码器
+type mockStreamDecoder struct {
+	chunks [][]canonical.CanonicalStreamEvent
+	flush  []canonical.CanonicalStreamEvent
+}
+
+// ProcessChunk 弹出下一个分片的事件
+func (d *mockStreamDecoder) ProcessChunk(rawChunk []byte) []canonical.CanonicalStreamEvent {
+	if len(d.chunks) == 0 {
+		return nil
+	}
+	events := d.chunks[0]
+	d.chunks = d.chunks[1:]
+	return events
+}
+
+// Flush 返回刷新事件
+func (d *mockStreamDecoder) Flush() []canonical.CanonicalStreamEvent {
+	return d.flush
+}
+
+// mockStreamEncoder 模拟流式编码器
+type mockStreamEncoder struct {
+	events [][]byte
+	flush  [][]byte
+}
+
+// EncodeEvent 返回编码后的事件
+func (e *mockStreamEncoder) EncodeEvent(event canonical.CanonicalStreamEvent) [][]byte {
+	if len(e.events) == 0 {
+		return nil
+	}
+	return e.events
+}
+
+// Flush 返回编码器刷新数据
+func (e *mockStreamEncoder) Flush() [][]byte {
+	return e.flush
+}
+
+func TestCanonicalStreamConverter_ProcessChunk(t *testing.T) {
+	event := canonical.NewMessageStartEvent("id-1", "gpt-4")
+	decoder := &mockStreamDecoder{
+		chunks: [][]canonical.CanonicalStreamEvent{{event}},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: test\n\n")},
+	}
+
+	converter := NewCanonicalStreamConverter(decoder, encoder)
+	result := converter.ProcessChunk([]byte("raw"))
+
+	assert.Len(t, result, 1)
+	assert.Equal(t, []byte("data: test\n\n"), result[0])
+}
+
+func TestCanonicalStreamConverter_WithMiddleware(t *testing.T) {
+	var records []string
+	event := canonical.NewMessageStartEvent("id-1", "gpt-4")
+	decoder := &mockStreamDecoder{
+		chunks: [][]canonical.CanonicalStreamEvent{{event}},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: ok\n\n")},
+	}
+
+	chain := NewMiddlewareChain()
+	chain.Use(&recordingMiddleware{name: "mw1", records: &records})
+	ctx := NewConversionContext(InterfaceTypeChat)
+
+	converter := NewCanonicalStreamConverterWithMiddleware(decoder, encoder, chain, *ctx, "openai", "anthropic")
+	result := converter.ProcessChunk([]byte("raw"))
+
+	assert.Len(t, result, 1)
+	assert.Equal(t, []string{"stream:mw1"}, records)
+	assert.Equal(t, []byte("data: ok\n\n"), result[0])
+}
+
+func TestCanonicalStreamConverter_Flush(t *testing.T) {
+	decoder := &mockStreamDecoder{
+		flush: []canonical.CanonicalStreamEvent{
+			canonical.NewMessageStopEvent(),
+		},
+	}
+	encoder := &mockStreamEncoder{
+		events: [][]byte{[]byte("data: stop\n\n")},
+		flush:  [][]byte{[]byte("data: flush\n\n")},
+	}
+
+	converter := NewCanonicalStreamConverter(decoder, encoder)
+	result := converter.Flush()
+
+	assert.Len(t, result, 2)
+	assert.Equal(t, []byte("data: stop\n\n"), result[0])
+	assert.Equal(t, []byte("data: flush\n\n"), result[1])
+}
+
+func TestCanonicalStreamConverter_EmptyDecoder(t *testing.T) {
+	decoder := &mockStreamDecoder{}
+	encoder := &mockStreamEncoder{}
+
+	converter := NewCanonicalStreamConverter(decoder, encoder)
+	result := converter.ProcessChunk([]byte("raw"))
+
+	assert.Nil(t, result)
+}