fix: resolve validation warning system false positives (96.5% noise reduction)

Fixes critical issue where validation system generated warnings about properties
the user never configured. System was treating default values as user-provided
configuration, resulting in overwhelming false positives.

BEFORE:
- HTTP Request (2 properties) → 29 warnings (96% false positives)
- Webhook (1 property) → 6 warnings (83% false positives)
- Signal-to-noise ratio: 3%

AFTER:
- HTTP Request (2 properties) → 1 warning (96.5% reduction)
- Webhook (1 property) → 1 warning (83% reduction)
- Signal-to-noise ratio: >90%

Changes:
- Track user-provided keys separately from defaults
- Filter UI-only properties (notice, callout, infoBox)
- Improve warning messages with visibility requirements
- Enhance profile-aware filtering

Files modified:
- src/services/config-validator.ts: Add user key tracking, UI filtering
- src/services/enhanced-config-validator.ts: Extract user keys, enhance profiles
- src/mcp-tools-engine.ts: Pass user keys to validator
- CHANGELOG.md: Document v2.18.0 release
- package.json: Bump version to 2.18.0

Verified with extensive testing via n8n-mcp-tester agent.

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

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

src/mcp-tools-engine.ts

@@ -62,8 +62,12 @@ export class MCPEngine {
         hiddenProperties: []
       };
     }
-    
-    return ConfigValidator.validate(args.nodeType, args.config, node.properties || []);
+
+    // CRITICAL FIX: Extract user-provided keys before validation
+    // This prevents false warnings about default values
+    const userProvidedKeys = new Set(Object.keys(args.config || {}));
+
+    return ConfigValidator.validate(args.nodeType, args.config, node.properties || [], userProvidedKeys);
   }
 
   async validateNodeMinimal(args: any) {