fix(sync): 限流检测漏掉了它真实的样子

上一版加了 429 识别，但点同步按钮报的还是 JSONDecodeError——因为检测
只看 `e.response.status_code` 和异常消息，而真实到达我们手里的异常两样
都不满足：garth 对着 429 调 .json()，抛出的 JSONDecodeError 里状态码已经
丢了，消息是无用的 "Expecting value: line 1 column 1"。

响应体「Rate limited」唯一幸存的地方是异常的 `doc` 属性，改为从那里取。

同时改正上一版的测试：它用 ValueError("Rate limited") 构造，那个形态现实中
根本不会出现，所以测试通过、检测却漏掉了每一个真实的限流。现在用真的
json.JSONDecodeError 构造，并先断言「光看消息不够」。

实测同步现在返回：
  RateLimited: Garmin 暂时限制了请求频率…令牌本身没有失效

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/services/garmin.py

@@ -111,11 +111,20 @@ def _note_rate_limit(user_id):
 
 
 def _is_rate_limited(e):
-    """429 from Garmin, however it happens to be dressed."""
+    """429 from Garmin, however it happens to be dressed.
+
+    Usually it arrives as a JSONDecodeError, because garth calls .json() on a
+    429 whose body is the plain text "Rate limited" — the status code is gone
+    by the time the exception reaches us, and the message is the useless
+    "Expecting value: line 1 column 1". What survives is the body itself, on
+    the exception's `doc` attribute, so that is where to look.
+    """
     response = getattr(e, "response", None)
     if response is not None and getattr(response, "status_code", None) == 429:
         return True
-    return "rate limit" in str(e).lower()
+
+    haystacks = [str(e), str(getattr(e, "doc", "") or "")]
+    return any("rate limit" in h.lower() for h in haystacks)
 
 
 class MFARequired(RuntimeError):