fix(line): return 200 for webhook verification requests without signature

LINE Platform sends POST {"events":[]} without an X-Line-Signature
header when the user clicks 'Verify' in the LINE Developers Console.
Both webhook.ts and monitor.ts rejected this with 400 'Missing
X-Line-Signature header', causing verification to fail.

Now detect the verification pattern (no signature + empty events array)
and return 200 OK immediately, while still requiring valid signatures
for all real webhook deliveries with non-empty events.

Fixes #16425

src/line/webhook.ts

@@ -39,13 +39,24 @@ export function createLineWebhookMiddleware(
   return async (req: Request, res: Response, _next: NextFunction): Promise<void> => {
     try {
       const signature = req.headers["x-line-signature"];
+      const rawBody = readRawBody(req);
 
+      // LINE webhook verification sends POST {"events":[]} without a
+      // signature header.  Return 200 immediately so the LINE Developers
+      // Console "Verify" button succeeds.
       if (!signature || typeof signature !== "string") {
+        if (rawBody) {
+          const body = parseWebhookBody(req, rawBody);
+          if (body && Array.isArray(body.events) && body.events.length === 0) {
+            logVerbose("line: webhook verification request (empty events, no signature) — 200 OK");
+            res.status(200).json({ status: "ok" });
+            return;
+          }
+        }
         res.status(400).json({ error: "Missing X-Line-Signature header" });
         return;
       }
 
-      const rawBody = readRawBody(req);
       if (!rawBody) {
         res.status(400).json({ error: "Missing raw request body for signature verification" });
         return;