feat: 添加 OpenAI Chat Completions 兼容端点

基于 @yulate 在 PR #648 (commit 0bb6a392) 的工作，解决了与最新
main 分支的合并冲突。

原始功能（@yulate）:
- 添加 /v1/chat/completions 和 /chat/completions 兼容端点
- 将 Chat Completions 请求转换为 Responses API 格式并转换回来
- 添加 API Key 直连转发支持
- 包含单元测试

Co-authored-by: yulate <yulate@users.noreply.github.com>

backend/internal/service/openai_chat_completions_test.go

@@ -0,0 +1,132 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestConvertChatCompletionsToResponses(t *testing.T) {
+	req := map[string]any{
+		"model": "gpt-4o",
+		"messages": []any{
+			map[string]any{
+				"role":    "user",
+				"content": "hello",
+			},
+			map[string]any{
+				"role": "assistant",
+				"tool_calls": []any{
+					map[string]any{
+						"id":   "call_1",
+						"type": "function",
+						"function": map[string]any{
+							"name":      "ping",
+							"arguments": "{}",
+						},
+					},
+				},
+			},
+			map[string]any{
+				"role":          "tool",
+				"tool_call_id":  "call_1",
+				"content":       "ok",
+				"response":      "ignored",
+				"response_time": 1,
+			},
+		},
+		"functions": []any{
+			map[string]any{
+				"name":        "ping",
+				"description": "ping tool",
+				"parameters":  map[string]any{"type": "object"},
+			},
+		},
+		"function_call": map[string]any{"name": "ping"},
+	}
+
+	converted, err := ConvertChatCompletionsToResponses(req)
+	require.NoError(t, err)
+	require.Equal(t, "gpt-4o", converted["model"])
+
+	input, ok := converted["input"].([]any)
+	require.True(t, ok)
+	require.Len(t, input, 3)
+
+	toolCall := findInputItemByType(input, "tool_call")
+	require.NotNil(t, toolCall)
+	require.Equal(t, "call_1", toolCall["call_id"])
+
+	toolOutput := findInputItemByType(input, "function_call_output")
+	require.NotNil(t, toolOutput)
+	require.Equal(t, "call_1", toolOutput["call_id"])
+
+	tools, ok := converted["tools"].([]any)
+	require.True(t, ok)
+	require.Len(t, tools, 1)
+
+	require.Equal(t, map[string]any{"name": "ping"}, converted["tool_choice"])
+}
+
+func TestConvertResponsesToChatCompletion(t *testing.T) {
+	resp := map[string]any{
+		"id":         "resp_123",
+		"model":      "gpt-4o",
+		"created_at": 1700000000,
+		"output": []any{
+			map[string]any{
+				"type": "message",
+				"role": "assistant",
+				"content": []any{
+					map[string]any{
+						"type": "output_text",
+						"text": "hi",
+					},
+				},
+			},
+		},
+		"usage": map[string]any{
+			"input_tokens":  2,
+			"output_tokens": 3,
+		},
+	}
+	body, err := json.Marshal(resp)
+	require.NoError(t, err)
+
+	converted, err := ConvertResponsesToChatCompletion(body)
+	require.NoError(t, err)
+
+	var chat map[string]any
+	require.NoError(t, json.Unmarshal(converted, &chat))
+	require.Equal(t, "chat.completion", chat["object"])
+
+	choices, ok := chat["choices"].([]any)
+	require.True(t, ok)
+	require.Len(t, choices, 1)
+
+	choice, ok := choices[0].(map[string]any)
+	require.True(t, ok)
+	message, ok := choice["message"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "hi", message["content"])
+
+	usage, ok := chat["usage"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, float64(2), usage["prompt_tokens"])
+	require.Equal(t, float64(3), usage["completion_tokens"])
+	require.Equal(t, float64(5), usage["total_tokens"])
+}
+
+func findInputItemByType(items []any, itemType string) map[string]any {
+	for _, item := range items {
+		itemMap, ok := item.(map[string]any)
+		if !ok {
+			continue
+		}
+		if itemMap["type"] == itemType {
+			return itemMap
+		}
+	}
+	return nil
+}