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/monitor.ts

@@ -322,8 +322,24 @@ export async function monitorLineProvider(
         const rawBody = await readLineWebhookRequestBody(req, LINE_WEBHOOK_MAX_BODY_BYTES);
         const signature = req.headers["x-line-signature"];
 
-        // Validate signature
+        // LINE webhook verification sends POST {"events":[]} without a
+        // signature header.  Return 200 so the LINE Developers Console
+        // "Verify" button succeeds.
         if (!signature || typeof signature !== "string") {
+          try {
+            const verifyBody = JSON.parse(rawBody) as WebhookRequestBody;
+            if (Array.isArray(verifyBody.events) && verifyBody.events.length === 0) {
+              logVerbose(
+                "line: webhook verification request (empty events, no signature) — 200 OK",
+              );
+              res.statusCode = 200;
+              res.setHeader("Content-Type", "application/json");
+              res.end(JSON.stringify({ status: "ok" }));
+              return;
+            }
+          } catch {
+            // Not valid JSON — fall through to the error below.
+          }
           logVerbose("line: webhook missing X-Line-Signature header");
           res.statusCode = 400;
           res.setHeader("Content-Type", "application/json");

src/line/webhook.test.ts

@@ -98,6 +98,48 @@ describe("createLineWebhookMiddleware", () => {
     expect(onEvents).not.toHaveBeenCalled();
   });
 
+  it("returns 200 for verification request (empty events, no signature)", async () => {
+    const onEvents = vi.fn(async () => {});
+    const secret = "secret";
+    const rawBody = JSON.stringify({ events: [] });
+    const middleware = createLineWebhookMiddleware({ channelSecret: secret, onEvents });
+
+    const req = {
+      headers: {},
+      body: rawBody,
+      // oxlint-disable-next-line typescript/no-explicit-any
+    } as any;
+    const res = createRes();
+
+    // oxlint-disable-next-line typescript/no-explicit-any
+    await middleware(req, res, {} as any);
+
+    expect(res.status).toHaveBeenCalledWith(200);
+    expect(res.json).toHaveBeenCalledWith({ status: "ok" });
+    expect(onEvents).not.toHaveBeenCalled();
+  });
+
+  it("rejects missing signature when events are non-empty", async () => {
+    const onEvents = vi.fn(async () => {});
+    const secret = "secret";
+    const rawBody = JSON.stringify({ events: [{ type: "message" }] });
+    const middleware = createLineWebhookMiddleware({ channelSecret: secret, onEvents });
+
+    const req = {
+      headers: {},
+      body: rawBody,
+      // oxlint-disable-next-line typescript/no-explicit-any
+    } as any;
+    const res = createRes();
+
+    // oxlint-disable-next-line typescript/no-explicit-any
+    await middleware(req, res, {} as any);
+
+    expect(res.status).toHaveBeenCalledWith(400);
+    expect(res.json).toHaveBeenCalledWith({ error: "Missing X-Line-Signature header" });
+    expect(onEvents).not.toHaveBeenCalled();
+  });
+
   it("rejects webhooks with signatures computed using wrong secret", async () => {
     const onEvents = vi.fn(async () => {});
     const correctSecret = "correct-secret";

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;