refactor: 优化 URL 路径拼接，修复 /v1 重复问题

## 主要变更

**核心修改**：
- 路由定义：/:protocol/v1/*path → /:protocol/*path
- proxy_handler：nativePath 直接使用 path 参数，不添加 /v1 前缀
- OpenAI 适配器：DetectInterfaceType 和 BuildUrl 去掉 /v1 前缀
- Anthropic 适配器：保持 /v1 前缀（Claude Code 兼容）

**URL 格式变化**：
- OpenAI: /openai/v1/chat/completions → /openai/chat/completions
- Anthropic: /anthropic/v1/messages (保持不变)

**base_url 配置**：
- OpenAI: 配置到版本路径，如 https://api.openai.com/v1
- Anthropic: 不配置版本路径，如 https://api.anthropic.com

## 测试验证

- 所有单元测试通过
- 所有集成测试通过
- 真实 API 测试验证成功
- 跨协议转换正常工作

## 文档更新

- 更新 backend/README.md URL 格式说明
- 同步 OpenSpec 规范文件

backend/tests/integration/conversion_test.go

@@ -77,7 +77,7 @@ func setupConversionTest(t *testing.T) (*gin.Engine, *gorm.DB, *httptest.Server)
 	r.Use(middleware.CORS())
 
 	// 代理路由
-	r.Any("/:protocol/v1/*path", proxyHandler.HandleProxy)
+	r.Any("/:protocol/*path", proxyHandler.HandleProxy)
 
 	// 管理路由
 	providers := r.Group("/api/providers")
@@ -177,7 +177,7 @@ func TestConversion_OpenAIToAnthropic_NonStream(t *testing.T) {
 	body, _ := json.Marshal(openaiReq)
 
 	w := httptest.NewRecorder()
-	req := httptest.NewRequest("POST", "/openai/v1/chat/completions", bytes.NewReader(body))
+	req := httptest.NewRequest("POST", "/openai/chat/completions", bytes.NewReader(body))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
 
@@ -202,7 +202,7 @@ func TestConversion_AnthropicToOpenAI_NonStream(t *testing.T) {
 		var req map[string]any
 		json.Unmarshal(body, &req)
 
-		assert.Equal(t, "/v1/chat/completions", r.URL.Path)
+		assert.Equal(t, "/chat/completions", r.URL.Path)
 		assert.Contains(t, r.Header.Get("Authorization"), "Bearer test-key")
 
 		resp := map[string]any{
@@ -262,7 +262,7 @@ func TestConversion_OpenAIToOpenAI_Passthrough(t *testing.T) {
 	r, _, upstream := setupConversionTest(t)
 
 	upstream.Config.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		assert.Equal(t, "/v1/chat/completions", r.URL.Path)
+		assert.Equal(t, "/chat/completions", r.URL.Path)
 
 		body, _ := io.ReadAll(r.Body)
 		var req map[string]any
@@ -284,7 +284,7 @@ func TestConversion_OpenAIToOpenAI_Passthrough(t *testing.T) {
 	body, _ := json.Marshal(reqBody)
 
 	w := httptest.NewRecorder()
-	req := httptest.NewRequest("POST", "/openai/v1/chat/completions", bytes.NewReader(body))
+	req := httptest.NewRequest("POST", "/openai/chat/completions", bytes.NewReader(body))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
 
@@ -364,7 +364,7 @@ func TestConversion_OpenAIToAnthropic_Stream(t *testing.T) {
 	body, _ := json.Marshal(openaiReq)
 
 	w := httptest.NewRecorder()
-	req := httptest.NewRequest("POST", "/openai/v1/chat/completions", bytes.NewReader(body))
+	req := httptest.NewRequest("POST", "/openai/chat/completions", bytes.NewReader(body))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
 
@@ -483,7 +483,7 @@ func TestConversion_ErrorResponse_Format(t *testing.T) {
 
 	// OpenAI 协议格式
 	w := httptest.NewRecorder()
-	req := httptest.NewRequest("POST", "/openai/v1/chat/completions", bytes.NewReader(body))
+	req := httptest.NewRequest("POST", "/openai/chat/completions", bytes.NewReader(body))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
 	assert.True(t, w.Code >= 400)
@@ -499,15 +499,15 @@ func TestConversion_OldRoutes_Return404(t *testing.T) {
 	req := httptest.NewRequest("POST", "/v1/chat/completions", strings.NewReader(`{"model":"test"}`))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
-	// Gin 路由不匹配返回 404
-	assert.Equal(t, 404, w.Code)
+	// Gin 路由匹配但协议不支持返回 400
+	assert.Equal(t, 400, w.Code)
 
 	// 旧 Anthropic 路由
 	w = httptest.NewRecorder()
 	req = httptest.NewRequest("POST", "/v1/messages", strings.NewReader(`{"model":"test"}`))
 	req.Header.Set("Content-Type", "application/json")
 	r.ServeHTTP(w, req)
-	assert.Equal(t, 404, w.Code)
+	assert.Equal(t, 400, w.Code)
 }
 
 // ============ Provider Protocol 字段测试 ============