fix: unify panic room code format and validate room join id

src/app/api/room/[id]/join/route.test.ts

@@ -74,11 +74,11 @@ describe("POST /api/room/[id]/join", () => {
   it("returns 404 when room not found", async () => {
     mockAtomicUpdate.mockResolvedValue(null);
 
-    const req = createRequest("/api/room/NONEXIST/join", {
+    const req = createRequest("/api/room/ABC123/join", {
       method: "POST",
       body: { userId: "user-1" },
     });
-    const ctx = createRouteContext({ id: "NONEXIST" });
+    const ctx = createRouteContext({ id: "ABC123" });
     const res = await POST(req, ctx);
     expect(res.status).toBe(404);
   });
@@ -92,4 +92,14 @@ describe("POST /api/room/[id]/join", () => {
     const res = await POST(req, ctx);
     expect(res.status).toBe(401);
   });
+
+  it("returns 400 when room id format is invalid", async () => {
+    const req = createRequest("/api/room/1234/join", {
+      method: "POST",
+      body: { userId: "user-1" },
+    });
+    const ctx = createRouteContext({ id: "1234" });
+    const res = await POST(req, ctx);
+    expect(res.status).toBe(400);
+  });
 });