fix(inbound): avoid HOL blocking on malformed-JID fallback bucket

Addresses Copilot review on PR #392.

The previous fallback `normalizedChatId || 'unknown'` funneled every
message with a missing or malformed chat id into a single global queue
on `postUpsertMutex`, head-of-line blocking unrelated malformed inputs
behind a shared bucket.

Prefer `msg.key?.id` over the constant: it is unique per message so
unrelated malformed inputs no longer serialize together, while valid
messages continue to hit the normalized chat-id path — preserving the
per-chat ordering guarantee where it actually matters. The CodeRabbit
concern about `msg.key.id` fragmenting the queue applied to the primary
key derivation (where ordering matters); here it is purely a fallback
for a corruption edge case where there is no chat to order by anyway.

src/Socket/chats.ts

@@ -1486,14 +1486,16 @@ export const makeChatsSocket = (config: SocketConfig) => {
 
 		// Use getChatId + jidNormalizedUser so the mutex key matches the chat-id
 		// scheme processMessage uses for chat updates (broadcasts target the
-		// participant). Falling back to a constant bucket avoids fragmenting the
-		// queue when the key is malformed.
-		// Compute the normalized value first then apply the fallback: a malformed
-		// but truthy rawChatId (e.g. missing `@`) makes jidNormalizedUser return
-		// '', which would otherwise pass through as an empty mutex key.
+		// participant). When getChatId/jidNormalizedUser yields nothing usable
+		// (missing or malformed JID), prefer a message-derived fallback over a
+		// single global 'unknown' bucket — that bucket would head-of-line block
+		// every malformed message behind a shared queue. msg.key.id is unique
+		// per message so unrelated malformed inputs no longer serialize together;
+		// for valid messages we still hit the normalized chat-id path, so the
+		// per-chat ordering guarantee is unchanged where it matters.
 		const rawChatId = getChatId(msg.key)
 		const normalizedChatId = rawChatId ? jidNormalizedUser(rawChatId) : ''
-		const postUpsertChatId = normalizedChatId || 'unknown'
+		const postUpsertChatId = normalizedChatId || msg.key?.id || 'unknown'
 
 		// Wrap in `postUpsertMutex(chatId)` (a SEPARATE keyed mutex from the outer
 		// `messageMutex` held by the inbound caller) so per-chat ordering of