fix(wechat): handle API errors instead of silently returning null h5_url

- Check HTTP status code, raise exception on non-200
- Raise exception if h5_url missing from response
- Fix description field to use .get() instead of direct key access
- Remove unreachable return None

Previously WeChat API errors (param errors, amount issues, etc.)
would return None, causing the frontend to show:
  https://token.opencomputing.cn/null
Now errors are properly raised and shown to the user.

unipay/providers/wechat.py

@@ -172,9 +172,14 @@ class WechatGateway(Gateway):
 				"Content-Type": "application/json"
 			}) as resp:
 				ret = await resp.json()
-				return ret.get("h5_url")
-
-		return None
+				if resp.status != 200:
+					err_code = ret.get("code", "UNKNOWN")
+					err_msg = ret.get("message", str(ret))
+					raise Exception(f"WeChat API error (HTTP {resp.status}): [{err_code}] {err_msg}")
+				h5_url = ret.get("h5_url")
+				if not h5_url:
+					raise Exception(f"WeChat API response missing h5_url: {json.dumps(ret, ensure_ascii=False)}")
+				return h5_url
 
 	# -----------------------------------------------------
 	async def refund(self, payload: Dict[str, Any]) -> Dict[str, Any]: