feat(translate): add Youdao as a new translation provider

The translate module previously only supported Baidu translation, and the
factory raised a bare RuntimeError for any other type. This change adds
Youdao Translation as a second provider and improves the factory's error
message.

Implementation details:
- New YoudaoTranslator class in translate/youdao/youdao_translate.py
- Implements Youdao's v3 SHA-256 signature scheme, including the
  truncate-input rule for queries longer than 20 characters
- Maps ISO 639-1 language codes to Youdao-specific codes
  (zh -> zh-CHS, zh-TW -> zh-CHT, others pass through)
- Differentiates network errors, API error codes, and empty translations
- factory.create_translator now lists the supported types in its
  RuntimeError message instead of failing silently
- Default config exposes youdao_translate_app_key and
  youdao_translate_app_secret

Adds 17 unit tests covering signature correctness, language code mapping,
input truncation edge cases, the full request/response flow, and factory
dispatch. All tests pass under Python 3.11.

translate/youdao/youdao_translate.py

@@ -0,0 +1,110 @@
+# -*- coding: utf-8 -*-
+"""
+Youdao translator implementation.
+
+Youdao Translation API v3 documentation:
+https://ai.youdao.com/DOCSIRMA/html/trans/api/wbfy/index.html
+
+Configuration keys (in config.json):
+    youdao_translate_app_key:    Application key from Youdao AI platform.
+    youdao_translate_app_secret: Application secret from Youdao AI platform.
+"""
+
+import time
+import uuid
+from hashlib import sha256
+
+import requests
+
+from config import conf
+from translate.translator import Translator
+
+
+class YoudaoTranslator(Translator):
+    """Youdao translator using the v3 signature scheme."""
+
+    API_URL = "https://openapi.youdao.com/api"
+
+    # Mapping from ISO 639-1 codes (used by the Translator interface)
+    # to Youdao-specific language codes.
+    # Reference: https://ai.youdao.com/DOCSIRMA/html/trans/api/wbfy/index.html
+    LANG_CODE_MAP = {
+        "": "auto",
+        "auto": "auto",
+        "zh": "zh-CHS",
+        "zh-CN": "zh-CHS",
+        "zh-TW": "zh-CHT",
+        "yue": "yue",  # Cantonese
+    }
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.app_key = conf().get("youdao_translate_app_key")
+        self.app_secret = conf().get("youdao_translate_app_secret")
+        if not self.app_key or not self.app_secret:
+            raise Exception("youdao translate app_key or app_secret not set")
+
+    def translate(self, query: str, from_lang: str = "", to_lang: str = "en") -> str:
+        if not query:
+            return ""
+
+        from_lang_code = self._convert_lang(from_lang) or "auto"
+        to_lang_code = self._convert_lang(to_lang) or "en"
+
+        salt = str(uuid.uuid4())
+        curtime = str(int(time.time()))
+        sign = self._build_sign(query, salt, curtime)
+
+        payload = {
+            "q": query,
+            "from": from_lang_code,
+            "to": to_lang_code,
+            "appKey": self.app_key,
+            "salt": salt,
+            "sign": sign,
+            "signType": "v3",
+            "curtime": curtime,
+        }
+        headers = {"Content-Type": "application/x-www-form-urlencoded"}
+
+        response = requests.post(self.API_URL, data=payload, headers=headers, timeout=10)
+        response.raise_for_status()
+        result = response.json()
+
+        error_code = result.get("errorCode", "0")
+        if error_code != "0":
+            raise Exception(
+                "youdao translate error: code={}, msg={}".format(
+                    error_code, result.get("msg", "")
+                )
+            )
+
+        translations = result.get("translation") or []
+        if not translations:
+            raise Exception("youdao translate returned empty translation")
+        return "\n".join(translations)
+
+    def _build_sign(self, query: str, salt: str, curtime: str) -> str:
+        """
+        Build the v3 signature.
+
+        sign = sha256(appKey + input + salt + curtime + appSecret),
+        where input = q if len(q) <= 20 else q[:10] + str(len(q)) + q[-10:].
+        """
+        input_str = self._truncate_input(query)
+        sign_str = self.app_key + input_str + salt + curtime + self.app_secret
+        return sha256(sign_str.encode("utf-8")).hexdigest()
+
+    @staticmethod
+    def _truncate_input(query: str) -> str:
+        length = len(query)
+        if length <= 20:
+            return query
+        return query[:10] + str(length) + query[-10:]
+
+    @classmethod
+    def _convert_lang(cls, lang: str) -> str:
+        """Convert ISO 639-1 language code to Youdao-specific code."""
+        if lang is None:
+            return "auto"
+        return cls.LANG_CODE_MAP.get(lang, lang)