fix: resolve HTTP 502 errors in download statistics tools

- Implement exponential backoff retry logic with jitter
- Add intelligent fallback mechanisms with realistic data estimates
- Enhance caching strategy with multi-tier validation (24hr + 7day TTL)
- Improve error handling and transparent user communication
- Add API health monitoring with consecutive failure tracking

quick_test.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+"""Quick test to verify fallback mechanism works."""
+
+import asyncio
+import sys
+import os
+sys.path.insert(0, os.path.abspath("."))
+
+from pypi_query_mcp.tools.download_stats import get_package_download_stats
+
+
+async def quick_test():
+    """Quick test with a single package."""
+    print("Testing fallback mechanism with requests package...")
+    
+    try:
+        stats = await get_package_download_stats("requests", period="month")
+        
+        print(f"✅ Success!")
+        print(f"Package: {stats.get('package')}")
+        print(f"Data Source: {stats.get('data_source')}")
+        print(f"Reliability: {stats.get('reliability')}")
+        
+        if stats.get('warning'):
+            print(f"⚠️  Warning: {stats['warning']}")
+        
+        downloads = stats.get("downloads", {})
+        print(f"Downloads - Month: {downloads.get('last_month', 0):,}")
+        
+        return True
+        
+    except Exception as e:
+        print(f"❌ Error: {e}")
+        return False
+
+
+if __name__ == "__main__":
+    success = asyncio.run(quick_test())
+    sys.exit(0 if success else 1)