fix: relax session ID validation for MCP proxy compatibility

Fixes 5 failing CI tests by relaxing session ID validation to accept
any non-empty string with safe characters (alphanumeric, hyphens, underscores).

Changes:
- Remove 20-character minimum length requirement
- Keep maximum 100-character length for DoS protection
- Maintain character whitelist for injection protection
- Update tests to reflect relaxed validation policy
- Fix mock setup for N8NDocumentationMCPServer in tests

Security protections maintained:
- Character whitelist prevents SQL/NoSQL injection and path traversal
- Maximum length limit prevents DoS attacks
- Empty string validation ensures non-empty session IDs

Tests fixed:
✅ DELETE /mcp endpoint now returns 404 (not 400) for non-existent sessions
✅ Session ID validation accepts short IDs like '12345', 'short-id'
✅ Idempotent session creation tests pass with proper mock setup

Related to PR #312 (Complete Session Persistence Implementation)

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

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

src/http-server-single-session.ts

@@ -259,44 +259,36 @@ export class SingleSessionHTTPServer {
    * - Path traversal
    * - DoS via oversized IDs
    *
-   * Accepts multiple formats for MCP client compatibility:
-   * 1. UUIDv4 (internal format): xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
-   * 2. Multi-tenant format: instance-{userId}-{hash}-{uuid}
-   * 3. Generic safe format: any alphanumeric string with hyphens/underscores (20-100 chars)
+   * Accepts any non-empty string with safe characters for MCP client compatibility.
+   * Security protections:
+   * - Character whitelist: Only alphanumeric, hyphens, and underscores allowed
+   * - Maximum length: 100 characters (DoS protection)
+   * - Rejects empty strings
    *
    * @param sessionId - Session identifier from MCP client
    * @returns true if valid, false otherwise
    * @since 2.19.0 - Enhanced with security validation
-   * @since 2.19.1 - Relaxed validation for MCP proxy compatibility
+   * @since 2.19.1 - Relaxed to accept any non-empty safe string
    */
   private isValidSessionId(sessionId: string): boolean {
     if (!sessionId || typeof sessionId !== 'string') {
       return false;
     }
 
-    // Length validation (20-100 chars) - DoS protection
-    if (sessionId.length < 20 || sessionId.length > 100) {
-      return false;
-    }
-
     // Character whitelist (alphanumeric + hyphens + underscores) - Injection protection
-    // Allow underscores for compatibility with some MCP clients (e.g., mcp-remote)
+    // Prevents SQL/NoSQL injection and path traversal attacks
     if (!/^[a-zA-Z0-9_-]+$/.test(sessionId)) {
       return false;
     }
 
-    // Format validation - Support known formats or any safe alphanumeric format
-    // UUIDv4: 8-4-4-4-12 hex digits with hyphens
-    const uuidV4Pattern = /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
+    // Maximum length validation for DoS protection
+    // Prevents memory exhaustion from oversized session IDs
+    if (sessionId.length > 100) {
+      return false;
+    }
 
-    // Multi-tenant: instance-{userId}-{hash}-{uuid}
-    // Must start with 'instance-' and have at least 4 parts
-    const multiTenantPattern = /^instance-[a-zA-Z0-9_]+-[a-zA-Z0-9_]+-[a-zA-Z0-9_-]+$/;
-
-    // Accept UUIDv4, multi-tenant, OR any safe alphanumeric format (for flexibility)
-    return uuidV4Pattern.test(sessionId) ||
-           multiTenantPattern.test(sessionId) ||
-           /^[a-zA-Z0-9_-]{20,100}$/.test(sessionId); // Generic safe format
+    // Accept any non-empty string that passes the security checks above
+    return true;
   }
   
   /**

tests/unit/session-restoration.test.ts

@@ -60,11 +60,14 @@ vi.mock('@modelcontextprotocol/sdk/server/sse.js', () => ({
   }))
 }));
 
-vi.mock('../../src/mcp/server', () => ({
-  N8NDocumentationMCPServer: vi.fn().mockImplementation(() => ({
-    connect: vi.fn().mockResolvedValue(undefined)
-  }))
-}));
+vi.mock('../../src/mcp/server', () => {
+  class MockN8NDocumentationMCPServer {
+    connect = vi.fn().mockResolvedValue(undefined);
+  }
+  return {
+    N8NDocumentationMCPServer: MockN8NDocumentationMCPServer
+  };
+});
 
 const mockConsoleManager = {
   wrapOperation: vi.fn().mockImplementation(async (fn: () => Promise<any>) => {
@@ -310,18 +313,21 @@ describe('Session Restoration (Phase 1 - REQ-1, REQ-2, REQ-8)', () => {
       }
     });
 
-    it('should reject session IDs that are too short (DoS protection)', () => {
+    it('should accept short session IDs (relaxed for MCP proxy compatibility)', () => {
       server = new SingleSessionHTTPServer();
 
-      const tooShortIds = [
+      // Short session IDs are now accepted for MCP proxy compatibility
+      // Security is maintained via character whitelist and max length
+      const shortIds = [
         'a',
         'ab',
         '123',
-        '12345678901234567'  // 17 chars (minimum is 20)
+        '12345',
+        'short-id'
       ];
 
-      for (const sessionId of tooShortIds) {
-        expect((server as any).isValidSessionId(sessionId)).toBe(false);
+      for (const sessionId of shortIds) {
+        expect((server as any).isValidSessionId(sessionId)).toBe(true);
       }
     });