feat: 配置 golangci-lint 静态分析并修复存量违规

- 新增 backend/.golangci.yml 配置 12 个 linter（forbidigo、errorlint、errcheck、staticcheck、revive、gocritic、gosec、bodyclose、noctx、nilerr、goimports、gocyclo）
- 新增 lefthook.yml 配置 pre-commit hook 自动运行 lint
- 修复存量代码违规：errors.Is/As 替换、zap.Error 替换、import 排序、errcheck 修复
- 更新 README 补充编码规范说明
- 归档 backend-code-lint 变更

backend/internal/conversion/openai/decoder.go

@@ -215,10 +215,16 @@ func decodeUserContent(content any) []canonical.ContentBlock {
 		var blocks []canonical.ContentBlock
 		for _, item := range v {
 			if m, ok := item.(map[string]any); ok {
-				t, _ := m["type"].(string)
+				t, ok := m["type"].(string)
+				if !ok {
+					continue
+				}
 				switch t {
 				case "text":
-					text, _ := m["text"].(string)
+					text, ok := m["text"].(string)
+					if !ok {
+						text = ""
+					}
 					blocks = append(blocks, canonical.NewTextBlock(text))
 				case "image_url":
 					blocks = append(blocks, canonical.ContentBlock{Type: "image"})
@@ -242,9 +248,9 @@ func decodeUserContent(content any) []canonical.ContentBlock {
 
 // contentPart 内容部分
 type contentPart struct {
-	Type     string
-	Text     string
-	Refusal  string
+	Type    string
+	Text    string
+	Refusal string
 }
 
 // decodeContentParts 解码内容部分
@@ -256,13 +262,22 @@ func decodeContentParts(content any) []contentPart {
 	var result []contentPart
 	for _, item := range parts {
 		if m, ok := item.(map[string]any); ok {
-			t, _ := m["type"].(string)
+			t, ok := m["type"].(string)
+			if !ok {
+				continue
+			}
 			switch t {
 			case "text":
-				text, _ := m["text"].(string)
+				text, ok := m["text"].(string)
+				if !ok {
+					text = ""
+				}
 				result = append(result, contentPart{Type: "text", Text: text})
 			case "refusal":
-				refusal, _ := m["refusal"].(string)
+				refusal, ok := m["refusal"].(string)
+				if !ok {
+					refusal = ""
+				}
 				result = append(result, contentPart{Type: "refusal", Refusal: refusal})
 			}
 		}
@@ -307,21 +322,33 @@ func decodeToolChoice(toolChoice any) *canonical.ToolChoice {
 			return canonical.NewToolChoiceAny()
 		}
 	case map[string]any:
-		t, _ := v["type"].(string)
+		t, ok := v["type"].(string)
+		if !ok {
+			return nil
+		}
 		switch t {
 		case "function":
 			if fn, ok := v["function"].(map[string]any); ok {
-				name, _ := fn["name"].(string)
+				name, ok := fn["name"].(string)
+				if !ok {
+					name = ""
+				}
 				return canonical.NewToolChoiceNamed(name)
 			}
 		case "custom":
 			if custom, ok := v["custom"].(map[string]any); ok {
-				name, _ := custom["name"].(string)
+				name, ok := custom["name"].(string)
+				if !ok {
+					name = ""
+				}
 				return canonical.NewToolChoiceNamed(name)
 			}
 		case "allowed_tools":
 			if at, ok := v["allowed_tools"].(map[string]any); ok {
-				mode, _ := at["mode"].(string)
+				mode, ok := at["mode"].(string)
+				if !ok {
+					mode = ""
+				}
 				if mode == "required" {
 					return canonical.NewToolChoiceAny()
 				}
@@ -443,7 +470,7 @@ func decodeDeprecatedFields(req *ChatCompletionRequest) {
 		case map[string]any:
 			if name, ok := v["name"].(string); ok {
 				req.ToolChoice = map[string]any{
-					"type": "function",
+					"type":     "function",
 					"function": map[string]any{"name": name},
 				}
 			}