fix: address Copilot code review findings (PR #137)

Fixes 3 critical issues identified in Copilot/Codex review:

## 1. Fixed Partial Config Override Bug (P1 - CRITICAL)
**File:** src/Socket/socket.ts
**Issue:** Using `||` operator caused partial configs to bypass defaults,
resulting in undefined fields (cleanupHour, intervalMs, etc.)

**Example bug:**
```typescript
// ❌ BEFORE - Broken!
sessionCleanupConfig: { autoCleanCorrupted: false }
// Results in: { autoCleanCorrupted: false } ← missing all other fields!
```

**Fix:** Proper object spread merge
```typescript
// ✅ AFTER - Correct!
const sessionCleanupConfig = {
  ...DEFAULT_SESSION_CLEANUP_CONFIG,
  ...(config.sessionCleanupConfig || {})
}
// Results in: { enabled: true, intervalMs: 86400000, ..., autoCleanCorrupted: false }
```

**Impact:** Prevents hot cleanup loops and undefined behavior

---

## 2. Fixed Nullish Coalescing Bug
**File:** src/Socket/messages-recv.ts
**Issue:** Using `||` instead of `??` caused undefined when config
exists but lacks specific field

**Fix:**
```typescript
// ❌ BEFORE
const autoCleanCorrupted = (sessionCleanupConfig || DEFAULT).autoCleanCorrupted
// If sessionCleanupConfig = {}, this returns undefined!

// ✅ AFTER
const autoCleanCorrupted = sessionCleanupConfig?.autoCleanCorrupted ?? DEFAULT.autoCleanCorrupted
```

**Impact:** Always gets correct default value

---

## 3. Improved API Usability
**File:** src/Types/Socket.ts
**Change:** `SessionCleanupConfig` → `Partial<SessionCleanupConfig>`

**Benefit:** Users can now override single fields without TypeScript errors
```typescript
// Now valid:
makeWASocket({
  sessionCleanupConfig: { autoCleanCorrupted: false }
})
```

## Validation
- ✅ TypeScript compilation: 0 errors
- ✅ Proper config merging tested
- ✅ Backward compatible

## Related
- PR #137 (Copilot review findings)
- Original fixes: PR #136

https://claude.ai/code/session_01SoNUGBEWbJwWWws3F2fuzh

src/Types/Socket.ts

@@ -254,6 +254,6 @@ export type SocketConfig = {
 	 */
 	enableUnifiedSession?: boolean
 
-	/** Session cleanup configuration (optional, uses DEFAULT_SESSION_CLEANUP_CONFIG if not provided) */
-	sessionCleanupConfig?: SessionCleanupConfig
+	/** Session cleanup configuration (optional, partial overrides merged with defaults) */
+	sessionCleanupConfig?: Partial<SessionCleanupConfig>
 }