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.ts

@@ -534,6 +534,17 @@ export class LdapService {
       errors: [],
     };
 
+    // DELIBERATE back-compat break + critical safety guard: an empty/undefined
+    // groupFilterDns now means "sync nothing", not "import everyone under
+    // baseDn". Returning here BEFORE any LDAP search and BEFORE the
+    // deactivation loop below is what prevents an empty selection from
+    // mass-deactivating every existing LDAP user (there would be no synced
+    // DNs to compare against, so every local LDAP user would look "removed").
+    // lastSyncAt is intentionally left untouched on this no-op path.
+    if (!config.groupFilterDns || config.groupFilterDns.length === 0) {
+      return result;
+    }
+
     const client = new Client(
       this.buildClientOptions(config.serverUrl, config.tlsRejectUnauthorized),
     );
@@ -673,8 +684,11 @@ export class LdapService {
    * Run the directory search for syncUsersForTenant, applying the selective
    * group/OU import filter (groupFilterDns) when one is configured.
    *
-   * - Empty groupFilterDns: single search of baseDn with sanitizedFilter
-   *   (identical to pre-filter behavior, backward compatible).
+   * - Empty groupFilterDns: DELIBERATE no-op — returns [] without ever
+   *   searching the directory. A selective sync now means "nothing selected
+   *   = nothing synced", not "import everyone under baseDn" (back-compat
+   *   break, see syncUsersForTenant's early-return guard which normally
+   *   short-circuits before this method is even reached).
    * - Non-empty groupFilterDns: split into OU DNs (used as extra search
    *   bases) and group DNs (matched via memberOf on the base search).
    *   Results are merged and deduped by entry dn.
@@ -686,12 +700,7 @@ export class LdapService {
     attributes: string[],
   ) {
     if (!config.groupFilterDns || config.groupFilterDns.length === 0) {
-      const { searchEntries } = await client.search(config.baseDn, {
-        filter: sanitizedFilter,
-        attributes,
-        scope: 'sub',
-      });
-      return searchEntries;
+      return [];
     }
 
     const ouBases = config.groupFilterDns.filter((dn) => /^ou=/i.test(dn));