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/openai/adapter_unified_test.go

@@ -18,40 +18,40 @@ func TestExtractUnifiedModelID(t *testing.T) {
 	a := NewAdapter()
 
 	t.Run("standard_path", func(t *testing.T) {
-		id, err := a.ExtractUnifiedModelID("/v1/models/openai/gpt-4")
+		id, err := a.ExtractUnifiedModelID("/models/openai/gpt-4")
 		require.NoError(t, err)
 		assert.Equal(t, "openai/gpt-4", id)
 	})
 
 	t.Run("multi_segment_path", func(t *testing.T) {
-		id, err := a.ExtractUnifiedModelID("/v1/models/azure/accounts/org/models/gpt-4")
+		id, err := a.ExtractUnifiedModelID("/models/azure/accounts/org/models/gpt-4")
 		require.NoError(t, err)
 		assert.Equal(t, "azure/accounts/org/models/gpt-4", id)
 	})
 
 	t.Run("single_segment", func(t *testing.T) {
-		id, err := a.ExtractUnifiedModelID("/v1/models/gpt-4")
+		id, err := a.ExtractUnifiedModelID("/models/gpt-4")
 		require.NoError(t, err)
 		assert.Equal(t, "gpt-4", id)
 	})
 
 	t.Run("non_model_path", func(t *testing.T) {
-		_, err := a.ExtractUnifiedModelID("/v1/chat/completions")
+		_, err := a.ExtractUnifiedModelID("/chat/completions")
 		require.Error(t, err)
 	})
 
 	t.Run("empty_suffix", func(t *testing.T) {
-		_, err := a.ExtractUnifiedModelID("/v1/models/")
+		_, err := a.ExtractUnifiedModelID("/models/")
 		require.Error(t, err)
 	})
 
 	t.Run("models_list_no_slash", func(t *testing.T) {
-		_, err := a.ExtractUnifiedModelID("/v1/models")
+		_, err := a.ExtractUnifiedModelID("/models")
 		require.Error(t, err)
 	})
 
 	t.Run("unrelated_path", func(t *testing.T) {
-		_, err := a.ExtractUnifiedModelID("/v1/other")
+		_, err := a.ExtractUnifiedModelID("/other")
 		require.Error(t, err)
 	})
 }
@@ -344,12 +344,12 @@ func TestIsModelInfoPath_UnifiedModelID(t *testing.T) {
 		path     string
 		expected bool
 	}{
-		{"simple_model_id", "/v1/models/gpt-4", true},
-		{"unified_model_id_with_slash", "/v1/models/openai/gpt-4", true},
-		{"models_list", "/v1/models", false},
-		{"models_list_trailing_slash", "/v1/models/", false},
-		{"chat_completions", "/v1/chat/completions", false},
-		{"deeply_nested", "/v1/models/azure/eastus/deployments/my-dept/models/gpt-4", true},
+		{"simple_model_id", "/models/gpt-4", true},
+		{"unified_model_id_with_slash", "/models/openai/gpt-4", true},
+		{"models_list", "/models", false},
+		{"models_list_trailing_slash", "/models/", false},
+		{"chat_completions", "/chat/completions", false},
+		{"deeply_nested", "/models/azure/eastus/deployments/my-dept/models/gpt-4", true},
 	}
 
 	for _, tt := range tests {