feat: implement Phase 6A integration tests (workflow validation)

Implemented comprehensive integration tests for workflow validation operations.

Test Coverage (12 scenarios):
- validate-workflow.test.ts: 12 test scenarios
  * Valid workflow with all 4 profiles (runtime, strict, ai-friendly, minimal)
  * Invalid workflow detection (bad node types, missing connections)
  * Selective validation (nodes only, connections only, expressions only)
  * Error handling (non-existent workflow, invalid parameters)
  * Response format verification

Infrastructure:
- Created node-repository utility for integration tests
- Provides singleton NodeRepository instance for validation tests
- Uses production nodes.db database

Test Results:
- All 83 integration tests passing (Phase 1-6A complete)
- Validation tests cover all 4 validation profiles
- Tests verify actual validation against real n8n instance

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

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

tests/integration/n8n-api/utils/node-repository.ts

@@ -0,0 +1,35 @@
+/**
+ * Node Repository Utility for Integration Tests
+ *
+ * Provides a singleton NodeRepository instance for integration tests
+ * that require validation or autofix functionality.
+ */
+
+import path from 'path';
+import { createDatabaseAdapter } from '../../../../src/database/database-adapter';
+import { NodeRepository } from '../../../../src/database/node-repository';
+
+let repositoryInstance: NodeRepository | null = null;
+
+/**
+ * Get or create NodeRepository instance
+ * Uses the production nodes.db database
+ */
+export async function getNodeRepository(): Promise<NodeRepository> {
+  if (repositoryInstance) {
+    return repositoryInstance;
+  }
+
+  const dbPath = path.join(process.cwd(), 'data/nodes.db');
+  const db = await createDatabaseAdapter(dbPath);
+  repositoryInstance = new NodeRepository(db);
+
+  return repositoryInstance;
+}
+
+/**
+ * Reset repository instance (useful for test cleanup)
+ */
+export function resetNodeRepository(): void {
+  repositoryInstance = null;
+}