fix(chats): remove null from LRUCache type to satisfy TypeScript constraint

CRITICAL FIX: Resolves TypeScript compilation error preventing npm install

ERROR:
  src/Socket/chats.ts(117,52): error TS2344: Type 'IAppStateSyncKeyData | null'
  does not satisfy the constraint '{}'. Type 'null' is not assignable to type '{}'.

ROOT CAUSE:
LRUCache v10+ has a generic constraint where value type V must satisfy constraint {}.
The type 'IAppStateSyncKeyData | null' includes null, which doesn't satisfy {}.

ANALYSIS:
The code NEVER caches null values (line 152 only calls set() if key is truthy),
so the | null in the type declaration was unnecessary and incorrect.

SOLUTION:
1. Remove | null from LRUCache type parameter (line 121)
2. Simplify getCachedAppStateSyncKey return (line 143)
3. Add documentation about type safety

VALIDATION:
✓ TypeScript error TS2344 resolved
✓ Logic unchanged: still only caches non-null values
✓ LRUCache.get() still returns undefined for missing keys
✓ Null cache poisoning prevention maintained (commit 4e05e62)

This fix enables successful npm install and build.

https://claude.ai/code/session_01NTVq3RHgGpgKL289JGvw55

src/Socket/chats.ts

@@ -113,8 +113,12 @@ export const makeChatsSocket = (config: SocketConfig) => {
 	 *
 	 * MEMORY SAFETY: Limited by DEFAULT_CACHE_MAX_KEYS.SIGNAL_STORE with 1-hour TTL.
 	 * Auto-purges expired entries to maintain memory bounds.
+	 *
+	 * TYPE SAFETY: Only successful lookups (non-null values) are cached.
+	 * Null/undefined values are NOT cached to prevent blocking newly arrived keys.
+	 * LRUCache.get() returns undefined for missing keys.
 	 */
-	const appStateSyncKeyCache = new LRUCache<string, proto.Message.IAppStateSyncKeyData | null>({
+	const appStateSyncKeyCache = new LRUCache<string, proto.Message.IAppStateSyncKeyData>({
 		max: DEFAULT_CACHE_MAX_KEYS.SIGNAL_STORE, // Use constant from Defaults (10,000)
 		ttl: DEFAULT_CACHE_TTLS.MSG_RETRY * 1000, // 1 hour TTL (convert seconds to ms)
 		ttlAutopurge: true, // Automatically remove expired entries
@@ -135,9 +139,8 @@ export const makeChatsSocket = (config: SocketConfig) => {
 		// Use get() directly to avoid race between has() and get() (Fix: Copilot C)
 		const cached = appStateSyncKeyCache.get(keyId)
 		if (cached !== undefined) {
-			// Null in cache means we explicitly cached a null (which we don't do anymore)
-			// Undefined means not in cache
-			return cached ?? undefined
+			// Cache hit - return the cached key
+			return cached
 		}
 
 		// Cache miss - fetch from database