fix(server): repair Pending lastSeen acknowledgement bookkeeping

[GoatTech-42]

Fixes #1535.

The reported bug: Pending.setPreviouslyAcknowledged() stored the return value of Array.prototype.push() (a length) instead of the array, so the next validation threw TypeError: this.lastSeen.some is not a function.

While writing a regression test for it, three more bugs surfaced in the same Pending acknowledgement path (src/server/chat.js, used for 1.19.1/1.19.2 servers with enforceSecureProfile) - each one alone breaks the flow before the reported line is even reached, so the PR fixes all four:

1. validateLastMessages called pending.get(messageSender)(messageSignature) - a curried call on a two-argument method. pending.get(...) returns a timestamp or undefined, so any first-time acknowledgement entry threw TypeError: pending.get(...) is not a function. Now pending.get(messageSender, messageSignature).

2. setPreviouslyAcknowledged (the reported bug) - now builds the array directly and only appends the rejected entry when it is non-empty:

   this.lastSeen = lastSeen.map(e => Object.values(e))
   const rejected = Object.values(lastRejected)
   if (rejected.length) this.lastSeen.push(rejected)

3. Signatures were compared by identity (===) in previouslyAcknowledged, acknowledge, and acknowledgePrior. Packet signatures are freshly parsed Buffers per message, so a client repeating a previously acknowledged entry could never match - exactly the case setPreviouslyAcknowledged exists to allow. Added a signaturesEqual helper that uses Buffer.equals when both sides are buffers. This also fixes acknowledge's findIndex returning -1 and splice(-1, 1) silently dropping the newest pending entry when a lastRejected signature didn't identity-match.

4. acknowledgePrior deleted this.m[a] (a sender's whole timestamp map) and called this.splice(0, i), which leaves the matched entry itself in the pending list. It now finds the match first, deletes only the covered per-signature entries, and splices index + 1 entries.

Regression tests in test/chatPendingTest.js drive the real chat_message handler across two packets: a repeated acknowledgement (re-parsed buffer, same bytes) must be accepted, a message the server never sent must still disconnect with multiplayer.disconnect.chat_validation_failed. On current master the first test fails on the very first packet (bug 1); with bug 1 fixed alone it fails on the second packet (the reported bug 2, then bug 3).

Lint (standard) passes; the new tests pass against the fixed code and fail against master.