fix: isGroupHandle incorrectly returns false for group chats

[shivshil]

Problem

isGroupHandle(identifier:guid:) in RPCPayloads.swift always returns false for group chats in RPC output (is_group field).

The function prefers identifier over guid:

let handle = identifier.isEmpty ? guid : identifier
return handle.contains(";+;") || handle.contains(";-;")

For group chats:

• identifier = bare GUID (e.g., 70012e07fc544289a74e87ecaf2ebe93) — no prefix
• guid = prefixed (e.g., any;+;70012e07fc544289a74e87ecaf2ebe93) — has ;+;

Since ChatInfo always populates identifier, it's never empty, so guid is never checked. The bare GUID doesn't contain ;+;, so the function returns false.

Additionally, the ;-; check is incorrect — that prefix indicates DMs (any;-;+14155551212), not groups.

Fix

func isGroupHandle(identifier: String, guid: String) -> Bool {
  return guid.contains(";+;") || identifier.contains(";+;")
}

Always check guid (which carries the prefix), also check identifier as a fallback, and only match the group prefix ;+;.

Verification

Tested with imsg rpc --json on macOS 15.x:

Before fix:

• Group chat 26: "is_group": false ❌
• DM chat 3: "is_group": false ✅

After fix:

• Group chat 26: "is_group": true ✅
• DM chat 3: "is_group": false ✅

This affects both chats.list and message notification payloads in the RPC server. The CLI watch and history commands (which use MessagePayload / OutputModels.swift) are unaffected since they don't emit is_group.