fix: skip node repository lookup for langchain nodes

Langchain AI nodes (tools, agents, chains) are already validated by specialized AI validators. Skipping the node repository lookup prevents "Unknown node type" errors when the database doesn't have langchain nodes, while still ensuring proper validation through AI-specific validators.

This fixes 7 integration test failures where valid AI tool configurations were incorrectly marked as invalid due to database lookup failures.

Resolves #265 (AI validation Phase 2 - remaining test failures)

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

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

src/services/workflow-validator.ts

@@ -399,6 +399,13 @@ export class WorkflowValidator {
         const nodeInfo = this.nodeRepository.getNode(normalizedType);
 
         if (!nodeInfo) {
+          // Skip validation for langchain nodes - they're validated by AI-specific validators
+          // This prevents database dependency issues while still ensuring proper validation
+          if (normalizedType.startsWith('nodes-langchain.')) {
+            // Langchain nodes are validated separately by validateAISpecificNodes()
+            continue;
+          }
+
           // Use NodeSimilarityService to find suggestions
           const suggestions = await this.similarityService.findSimilarNodes(node.type, 3);