fix(adapter): do not let volatile packets break connection state recovery

[chuanghiduoc]

Problem

When connectionStateRecovery is enabled, a volatile event whose last argument is a string breaks the recovery of the client that received it.

Reproduction (server + client from current main):

const io = new Server(httpServer, { connectionStateRecovery: {} });

io.on("connection", (socket) => {
  // ...
});

// after a normal broadcast has initialized the offset chain:
io.volatile.emit("status", "connected");

The client then disconnects and reconnects: socket.recovered is false, and every packet emitted during the disconnection is silently lost.

With the same scenario minus the io.volatile.emit(...) line, socket.recovered is true.

Cause

The two sides of the offset mechanism disagree about volatile packets:

• server side (SessionAwareAdapter.broadcast()): a volatile EVENT packet gets no offset appended
• client side (emitEvent()): any trailing string argument of a received event is stored as the offset

So after io.volatile.emit("status", "connected"), the client stores the literal string "connected" as its offset. On reconnection, restoreSession() does findIndex(packet.id === "connected"), which fails, and the session is dropped.

This also affects the cluster adapters, which share the same condition in addOffsetIfNecessary(). I have deliberately not touched them here, because their packet storage/replay lives in the external adapter packages and needs to be changed in sync.

Fix

In the in-memory adapter, volatile EVENT packets are now part of the offset chain like any other EVENT packet (they get an offset, and they are stored), but they are excluded from the replay in restoreSession(). Their semantics are unchanged - a volatile packet is still never delivered late.

Testing

• new test in packages/socket.io-adapter/test/index.ts: session restore still succeeds with the volatile packet's offset, and the volatile packet is still excluded from missedPackets
• npm test in packages/socket.io-adapter: 49 passing (48 before)
• full suite in packages/socket.io: 216 passing

I also verified end-to-end with a script against the built packages: before the fix recovered: false / no replay; after the fix recovered: true / non-volatile packets replayed in order / volatile packets not replayed.