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/middleware_test.go

@@ -0,0 +1,85 @@
+package conversion
+
+import (
+	"errors"
+	"testing"
+
+	"nex/backend/internal/conversion/canonical"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// recordingMiddleware 记录调用顺序的中间件
+type recordingMiddleware struct {
+	name    string
+	records *[]string
+	err     error
+}
+
+func (m *recordingMiddleware) Intercept(req *canonical.CanonicalRequest, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalRequest, error) {
+	*m.records = append(*m.records, m.name)
+	if m.err != nil {
+		return nil, m.err
+	}
+	return req, nil
+}
+
+func (m *recordingMiddleware) InterceptStreamEvent(event *canonical.CanonicalStreamEvent, clientProtocol, providerProtocol string, ctx *ConversionContext) (*canonical.CanonicalStreamEvent, error) {
+	*m.records = append(*m.records, "stream:"+m.name)
+	if m.err != nil {
+		return nil, m.err
+	}
+	return event, nil
+}
+
+func TestMiddlewareChain_Empty(t *testing.T) {
+	chain := NewMiddlewareChain()
+	req := &canonical.CanonicalRequest{Model: "test"}
+	ctx := NewConversionContext(InterfaceTypeChat)
+
+	result, err := chain.Apply(req, "a", "b", ctx)
+	assert.NoError(t, err)
+	assert.Equal(t, "test", result.Model)
+}
+
+func TestMiddlewareChain_Order(t *testing.T) {
+	var records []string
+	chain := NewMiddlewareChain()
+	chain.Use(&recordingMiddleware{name: "first", records: &records})
+	chain.Use(&recordingMiddleware{name: "second", records: &records})
+	chain.Use(&recordingMiddleware{name: "third", records: &records})
+
+	req := &canonical.CanonicalRequest{Model: "test"}
+	ctx := NewConversionContext(InterfaceTypeChat)
+	_, err := chain.Apply(req, "a", "b", ctx)
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"first", "second", "third"}, records)
+}
+
+func TestMiddlewareChain_ErrorInterrupt(t *testing.T) {
+	var records []string
+	chain := NewMiddlewareChain()
+	chain.Use(&recordingMiddleware{name: "first", records: &records})
+	chain.Use(&recordingMiddleware{name: "second", records: &records, err: errors.New("中断")})
+	chain.Use(&recordingMiddleware{name: "third", records: &records})
+
+	req := &canonical.CanonicalRequest{Model: "test"}
+	ctx := NewConversionContext(InterfaceTypeChat)
+	_, err := chain.Apply(req, "a", "b", ctx)
+	assert.Error(t, err)
+	assert.Equal(t, "中断", err.Error())
+	assert.Equal(t, []string{"first", "second"}, records)
+}
+
+func TestMiddlewareChain_ApplyStreamEvent(t *testing.T) {
+	var records []string
+	chain := NewMiddlewareChain()
+	chain.Use(&recordingMiddleware{name: "mw1", records: &records})
+
+	event := canonical.NewMessageStartEvent("id", "model")
+	ctx := NewConversionContext(InterfaceTypeChat)
+	result, err := chain.ApplyStreamEvent(&event, "a", "b", ctx)
+	assert.NoError(t, err)
+	assert.Equal(t, canonical.EventMessageStart, result.Type)
+	assert.Equal(t, []string{"stream:mw1"}, records)
+}