feat: add error handling for fetch operations in i18n

- Add new error messages for fetch operations
- Rename `errInvalidLocationFormat` to `errInvalidLocationFormatKey`
- Use `errors.New` instead of `fmt.Errorf` for fetch errors
- Update Chinese locale with fetch-related error messages

internal/i18n/locales/zh.json

@@ -481,5 +481,15 @@
   "bedrock_converse_failed": "模型 %s 的 bedrock converse 失败：%w",
   "bedrock_unexpected_response_type": "意外的响应类型：%T",
   "bedrock_empty_response_content": "空响应内容",
-  "bedrock_unexpected_content_block_type": "意外的内容块类型：%T"
+  "bedrock_unexpected_content_block_type": "意外的内容块类型：%T",
+  "fetch_unknown_operation": "fetch：未知操作 %q（支持：get）",
+  "fetch_content_not_utf8": "fetch：内容不是有效的 UTF-8 文本",
+  "fetch_content_null_bytes": "fetch：内容包含空字节",
+  "fetch_error_create_request": "fetch：创建请求时出错：%v",
+  "fetch_error_fetching_url": "fetch：获取 URL 时出错：%v",
+  "fetch_http_error": "fetch：HTTP 错误：%d - %s",
+  "fetch_content_too_large": "fetch：内容过大：%d 字节（最大 %d 字节）",
+  "fetch_unsupported_content_type": "fetch：不支持的内容类型 %q - 仅允许文本内容",
+  "fetch_error_reading_response": "fetch：读取响应时出错：%v",
+  "fetch_content_exceeds_limit": "fetch：内容过大：超过 %d 字节"
 }

internal/plugins/ai/gemini/gemini.go

@@ -31,7 +31,7 @@ const (
 )
 
 const (
-	errInvalidLocationFormat = "gemini_invalid_location_format"
+	errInvalidLocationFormatKey = "gemini_invalid_location_format"
 	locationSeparator        = "/"
 	langCodeSeparator        = "_"
 	langCodeNormalizedSep    = "-"
@@ -232,7 +232,7 @@ func (o *Client) buildGenerateContentConfig(opts *domain.ChatOptions) (*genai.Ge
 					RetrievalConfig: &genai.RetrievalConfig{LanguageCode: loc},
 				}
 			} else {
-				return nil, fmt.Errorf(i18n.T(errInvalidLocationFormat), loc)
+				return nil, fmt.Errorf(i18n.T(errInvalidLocationFormatKey), loc)
 			}
 		}
 	}

internal/plugins/template/fetch.go

@@ -72,11 +72,11 @@ func (p *FetchPlugin) validateTextContent(content []byte) error {
 	debugf("Fetch: validating content length=%d bytes", len(content))
 
 	if !utf8.Valid(content) {
-		return fmt.Errorf(i18n.T("fetch_content_not_utf8"))
+		return errors.New(i18n.T("fetch_content_not_utf8"))
 	}
 
 	if bytes.Contains(content, []byte{0}) {
-		return fmt.Errorf(i18n.T("fetch_content_null_bytes"))
+		return errors.New(i18n.T("fetch_content_null_bytes"))
 	}
 
 	debugf("Fetch: content validation successful")