fix(mcp-gateway): address critical issues from deep review

Frontend:
- Fix debounce race condition in UserListTable search handler
- Use useRef to properly track and cleanup timeout between keystrokes

Backend (LLM Gateway):
- Add thread-safe double-checked locking for global singletons
  (providers, circuit registry, cost tracker)
- Fix Redis URL parsing with proper urlparse validation
- Add explicit error handling for malformed Redis URLs
- Document circuit breaker state transition safety

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/admin/users/UserListTable.tsx

@@ -5,7 +5,7 @@
 
 'use client';
 
-import { useState, useCallback } from 'react';
+import { useState, useCallback, useRef, useEffect } from 'react';
 import { format } from 'date-fns';
 import { Check, X } from 'lucide-react';
 import {
@@ -61,15 +61,28 @@ export function UserListTable({
   currentUserId,
 }: UserListTableProps) {
   const [searchValue, setSearchValue] = useState('');
+  const searchTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);
 
-  // Debounce search
+  // Cleanup timeout on unmount
+  useEffect(() => {
+    return () => {
+      if (searchTimeoutRef.current) {
+        clearTimeout(searchTimeoutRef.current);
+      }
+    };
+  }, []);
+
+  // Debounce search with proper cleanup
   const handleSearchChange = useCallback(
     (value: string) => {
       setSearchValue(value);
-      const timeoutId = setTimeout(() => {
+      // Clear previous timeout to prevent stale searches
+      if (searchTimeoutRef.current) {
+        clearTimeout(searchTimeoutRef.current);
+      }
+      searchTimeoutRef.current = setTimeout(() => {
         onSearch(value);
       }, 300);
-      return () => clearTimeout(timeoutId);
     },
     [onSearch]
   );