feat(260728-lih): make LDAP sync strictly selective (empty selection = no-op)

- collectSearchEntries() returns [] on empty/undefined groupFilterDns
  instead of scanning the whole baseDn subtree
- syncUsersForTenant() early-returns an empty successful result before
  any LDAP search or the deactivation loop when groupFilterDns is empty,
  so an empty selection can never mass-deactivate existing LDAP users
- Updated exclude-list tests to use a non-empty groupFilterDns; added a
  dedicated no-op test proving empty selection performs zero search/
  create/update/deactivate operations

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/ldap/ldap.service.spec.ts

@@ -34,7 +34,10 @@ describe('LdapService.syncUsersForTenant — per-user exclude list', () => {
     serverUrl: 'ldap://example',
     baseDn: 'dc=example,dc=com',
     searchFilter: '(objectClass=person)',
-    groupFilterDns: [] as string[],
+    // Non-empty selection: an empty groupFilterDns is now a no-op (see the
+    // dedicated "empty selection no-op" describe block below), so these
+    // exclude-list tests need a real selection to exercise the search path.
+    groupFilterDns: ['ou=people,dc=example,dc=com'] as string[],
     userExcludeList: [] as string[],
     fieldMappings: [
       { ldapField: 'sAMAccountName', tesseraField: 'username' },
@@ -115,6 +118,55 @@ describe('LdapService.syncUsersForTenant — per-user exclude list', () => {
   });
 });
 
+describe('LdapService.syncUsersForTenant — empty selection no-op', () => {
+  let service: LdapService;
+  let prisma: any;
+  let userService: any;
+
+  const emptySelectionConfig = {
+    id: 'cfg1',
+    tenantId: 't1',
+    serverUrl: 'ldap://example',
+    baseDn: 'dc=example,dc=com',
+    searchFilter: '(objectClass=person)',
+    groupFilterDns: [] as string[],
+    userExcludeList: [] as string[],
+    fieldMappings: [
+      { ldapField: 'sAMAccountName', tesseraField: 'username' },
+    ],
+  };
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockBind.mockResolvedValue(undefined);
+    mockUnbind.mockResolvedValue(undefined);
+    prisma = {
+      user: {
+        findFirst: vi.fn().mockResolvedValue(null),
+        findMany: vi.fn().mockResolvedValue([{ id: 'u-existing', ldapDn: 'cn=existing' }]),
+        update: vi.fn().mockResolvedValue({}),
+      },
+      ldapConfig: { update: vi.fn().mockResolvedValue({}) },
+    };
+    userService = { create: vi.fn().mockResolvedValue({}) };
+    service = new LdapService(prisma, userService);
+  });
+
+  it('creates nobody and deactivates nobody when groupFilterDns is empty', async () => {
+    const result = await service.syncUsersForTenant(
+      emptySelectionConfig as any,
+      't1',
+    );
+
+    expect(result).toEqual({ created: 0, updated: 0, deactivated: 0, errors: [] });
+    expect(mockSearch).not.toHaveBeenCalled();
+    expect(mockBind).not.toHaveBeenCalled();
+    expect(userService.create).not.toHaveBeenCalled();
+    expect(prisma.user.update).not.toHaveBeenCalled();
+    expect(prisma.ldapConfig.update).not.toHaveBeenCalled();
+  });
+});
+
 describe('LdapService — individual user search & import (dedup)', () => {
   let service: LdapService;
   let prisma: any;