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/adapter_test.go

@@ -0,0 +1,139 @@
+package openai
+
+import (
+	"encoding/json"
+	"testing"
+
+	"nex/backend/internal/conversion"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAdapter_ProtocolName(t *testing.T) {
+	a := NewAdapter()
+	assert.Equal(t, "openai", a.ProtocolName())
+}
+
+func TestAdapter_SupportsPassthrough(t *testing.T) {
+	a := NewAdapter()
+	assert.True(t, a.SupportsPassthrough())
+}
+
+func TestAdapter_DetectInterfaceType(t *testing.T) {
+	a := NewAdapter()
+
+	tests := []struct {
+		name     string
+		path     string
+		expected conversion.InterfaceType
+	}{
+		{"聊天补全", "/v1/chat/completions", conversion.InterfaceTypeChat},
+		{"模型列表", "/v1/models", conversion.InterfaceTypeModels},
+		{"模型详情", "/v1/models/gpt-4", conversion.InterfaceTypeModelInfo},
+		{"嵌入接口", "/v1/embeddings", conversion.InterfaceTypeEmbeddings},
+		{"重排序接口", "/v1/rerank", conversion.InterfaceTypeRerank},
+		{"未知路径", "/v1/unknown", conversion.InterfaceTypePassthrough},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := a.DetectInterfaceType(tt.path)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestAdapter_BuildUrl(t *testing.T) {
+	a := NewAdapter()
+
+	tests := []struct {
+		name         string
+		nativePath   string
+		interfaceType conversion.InterfaceType
+		expected     string
+	}{
+		{"聊天", "/v1/chat/completions", conversion.InterfaceTypeChat, "/v1/chat/completions"},
+		{"模型", "/v1/models", conversion.InterfaceTypeModels, "/v1/models"},
+		{"嵌入", "/v1/embeddings", conversion.InterfaceTypeEmbeddings, "/v1/embeddings"},
+		{"重排序", "/v1/rerank", conversion.InterfaceTypeRerank, "/v1/rerank"},
+		{"默认透传", "/v1/other", conversion.InterfaceTypePassthrough, "/v1/other"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := a.BuildUrl(tt.nativePath, tt.interfaceType)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestAdapter_BuildHeaders(t *testing.T) {
+	a := NewAdapter()
+
+	t.Run("基本头", func(t *testing.T) {
+		provider := conversion.NewTargetProvider("https://api.openai.com", "sk-test123", "gpt-4")
+		headers := a.BuildHeaders(provider)
+		assert.Equal(t, "Bearer sk-test123", headers["Authorization"])
+		assert.Equal(t, "application/json", headers["Content-Type"])
+		_, hasOrg := headers["OpenAI-Organization"]
+		assert.False(t, hasOrg)
+	})
+
+	t.Run("带组织", func(t *testing.T) {
+		provider := conversion.NewTargetProvider("https://api.openai.com", "sk-test123", "gpt-4")
+		provider.AdapterConfig["organization"] = "org-abc"
+		headers := a.BuildHeaders(provider)
+		assert.Equal(t, "org-abc", headers["OpenAI-Organization"])
+	})
+}
+
+func TestAdapter_SupportsInterface(t *testing.T) {
+	a := NewAdapter()
+
+	tests := []struct {
+		name         string
+		interfaceType conversion.InterfaceType
+		expected     bool
+	}{
+		{"聊天", conversion.InterfaceTypeChat, true},
+		{"模型", conversion.InterfaceTypeModels, true},
+		{"模型详情", conversion.InterfaceTypeModelInfo, true},
+		{"嵌入", conversion.InterfaceTypeEmbeddings, true},
+		{"重排序", conversion.InterfaceTypeRerank, true},
+		{"透传", conversion.InterfaceTypePassthrough, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := a.SupportsInterface(tt.interfaceType)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestAdapter_EncodeError_InvalidInput(t *testing.T) {
+	a := NewAdapter()
+	convErr := conversion.NewConversionError(conversion.ErrorCodeInvalidInput, "参数无效")
+
+	body, statusCode := a.EncodeError(convErr)
+	require.Equal(t, 500, statusCode)
+
+	var resp ErrorResponse
+	require.NoError(t, json.Unmarshal(body, &resp))
+	assert.Equal(t, "参数无效", resp.Error.Message)
+	assert.Equal(t, "invalid_request_error", resp.Error.Type)
+}
+
+func TestAdapter_EncodeError_ServerError(t *testing.T) {
+	a := NewAdapter()
+	convErr := conversion.NewConversionError(conversion.ErrorCodeStreamStateError, "流状态错误")
+
+	body, statusCode := a.EncodeError(convErr)
+	require.Equal(t, 500, statusCode)
+
+	var resp ErrorResponse
+	require.NoError(t, json.Unmarshal(body, &resp))
+	assert.Equal(t, "server_error", resp.Error.Type)
+	assert.Equal(t, "流状态错误", resp.Error.Message)
+}