test: add Phase 4 database integration tests (partial)

- Add comprehensive test utilities for database testing
- Implement connection management tests for in-memory and file databases
- Add transaction tests including nested transactions and savepoints
- Test database lifecycle, error handling, and performance
- Include tests for WAL mode, connection pooling, and constraints

Part of Phase 4: Integration Testing

tests/integration/mcp-protocol/test-helpers.ts

@@ -0,0 +1,101 @@
+import { Server } from '@modelcontextprotocol/sdk/server/index.js';
+import { Transport } from '@modelcontextprotocol/sdk';
+import { 
+  CallToolRequestSchema, 
+  ListToolsRequestSchema,
+  InitializeRequestSchema,
+} from '@modelcontextprotocol/sdk/types.js';
+import { N8NDocumentationMCPServer } from '../../../src/mcp/server';
+
+export class TestableN8NMCPServer {
+  private mcpServer: N8NDocumentationMCPServer;
+  private server: Server;
+  private transport?: Transport;
+
+  constructor() {
+    this.server = new Server({
+      name: 'n8n-documentation-mcp',
+      version: '1.0.0'
+    }, {
+      capabilities: {
+        tools: {}
+      }
+    });
+    
+    this.mcpServer = new N8NDocumentationMCPServer();
+    this.setupHandlers();
+  }
+
+  private setupHandlers() {
+    // Initialize handler
+    this.server.setRequestHandler(InitializeRequestSchema, async () => {
+      return {
+        protocolVersion: '2024-11-05',
+        capabilities: {
+          tools: {}
+        },
+        serverInfo: {
+          name: 'n8n-documentation-mcp',
+          version: '1.0.0'
+        }
+      };
+    });
+
+    // List tools handler
+    this.server.setRequestHandler(ListToolsRequestSchema, async () => {
+      const tools = await this.mcpServer.executeTool('tools/list', {});
+      return tools;
+    });
+
+    // Call tool handler
+    this.server.setRequestHandler(CallToolRequestSchema, async (request) => {
+      try {
+        const result = await this.mcpServer.executeTool(request.params.name, request.params.arguments || {});
+        
+        // Convert result to content array if needed
+        if (Array.isArray(result) && result.length > 0 && result[0].content) {
+          return result;
+        }
+        
+        return {
+          content: [
+            {
+              type: 'text',
+              text: typeof result === 'string' ? result : JSON.stringify(result, null, 2)
+            }
+          ]
+        };
+      } catch (error: any) {
+        return {
+          content: [
+            {
+              type: 'text',
+              text: `Error: ${error.message}`
+            }
+          ],
+          isError: true
+        };
+      }
+    });
+  }
+
+  async initialize(): Promise<void> {
+    // The MCP server initializes its database lazily
+    // We can trigger initialization by calling executeTool
+    try {
+      await this.mcpServer.executeTool('get_database_statistics', {});
+    } catch (error) {
+      // Ignore errors, we just want to trigger initialization
+    }
+  }
+
+  async connectToTransport(transport: Transport): Promise<void> {
+    this.transport = transport;
+    await this.server.connect(transport);
+  }
+
+  async close(): Promise<void> {
+    // The server handles closing the transport
+    await this.mcpServer.shutdown();
+  }
+}