feat: 系统性改进后端测试体系

- 新增 6 个测试场景 (config load pipe, handler errors, service aggregation, engine degradation, openai decoder edges, negative tests)
- 更新测试工具规格 (mockgen, in-memory SQLite)
- 覆盖率目标从 >80% 提升至 >85%
- 新增 test-unit 和 test-integration Makefile 命令
- 新增死代码清理和 mockgen 需求
- 归档变更至 openspec/changes/archive/2026-04-22-improve-backend-testing/

backend/internal/conversion/openai/decoder_test.go

@@ -409,3 +409,25 @@ func TestDecodeResponse_Refusal(t *testing.T) {
 	}
 	assert.True(t, found)
 }
+
+func TestDecodeRequest_AssistantContentArray(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-4",
+		"messages": [
+			{"role": "user", "content": "hello"},
+			{
+				"role": "assistant",
+				"content": [{"type": "text", "text": "hello back"}]
+			}
+		]
+	}`)
+
+	req, err := decodeRequest(body)
+	require.NoError(t, err)
+	assert.Len(t, req.Messages, 2)
+	assistantMsg := req.Messages[1]
+	assert.Equal(t, canonical.RoleAssistant, assistantMsg.Role)
+	assert.Len(t, assistantMsg.Content, 1)
+	assert.Equal(t, "text", assistantMsg.Content[0].Type)
+	assert.Equal(t, "hello back", assistantMsg.Content[0].Text)
+}