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/internal/conversion/engine_supplemental_test.go

@@ -58,7 +58,7 @@ func TestEngine_Use(t *testing.T) {
 	_ = engine.RegisterAdapter(providerAdapter)
 
 	_, err := engine.ConvertHttpRequest(HTTPRequestSpec{
-		URL: "/v1/chat/completions", Method: "POST", Body: []byte(`{}`),
+		URL: "/chat/completions", Method: "POST", Body: []byte(`{}`),
 	}, "client", "provider", NewTargetProvider("https://example.com", "key", "model"))
 	require.NoError(t, err)
 	assert.True(t, called)
@@ -75,7 +75,7 @@ func TestConvertHttpRequest_DecodeError(t *testing.T) {
 	_ = engine.RegisterAdapter(newMockAdapter("provider", false))
 
 	_, err := engine.ConvertHttpRequest(HTTPRequestSpec{
-		URL: "/v1/chat/completions", Method: "POST", Body: []byte(`{}`),
+		URL: "/chat/completions", Method: "POST", Body: []byte(`{}`),
 	}, "client", "provider", NewTargetProvider("", "", ""))
 	assert.Error(t, err)
 }
@@ -91,7 +91,7 @@ func TestConvertHttpRequest_EncodeError(t *testing.T) {
 	_ = engine.RegisterAdapter(providerAdapter)
 
 	_, err := engine.ConvertHttpRequest(HTTPRequestSpec{
-		URL: "/v1/chat/completions", Method: "POST", Body: []byte(`{}`),
+		URL: "/chat/completions", Method: "POST", Body: []byte(`{}`),
 	}, "client", "provider", NewTargetProvider("", "", ""))
 	assert.Error(t, err)
 }
@@ -224,7 +224,7 @@ func TestConvertHttpRequest_ModelsInterface_Passthrough(t *testing.T) {
 
 	body := []byte(`{"object":"list","data":[]}`)
 	result, err := engine.ConvertHttpRequest(HTTPRequestSpec{
-		URL: "/v1/models", Method: "GET", Body: body,
+		URL: "/models", Method: "GET", Body: body,
 	}, "client", "provider", NewTargetProvider("https://example.com", "key", ""))
 	require.NoError(t, err)
 	assert.Equal(t, body, result.Body)