refactor(html-embed): extract the admin-gate strip into one tested helper (#90)

The 4-step html-embed gate (feature-enabled AND role-allowed -> stripHtmlEmbedNodes)
was replicated across call-sites, pinned only by brittle source-regex tests. Add
stripHtmlEmbedIfNotAllowed(json, {featureEnabled, role, onStrip}) and migrate the
5 plain strip-all sites (collab handler, page create+duplicate, both import paths,
transclusion) to it, each keeping its own feature/role resolve + log via onStrip.
Left the 2 sites with different semantics: persistence.extension (#29 preserve-
admin) and share.service (feature-only kill-switch, no role gate). Real unit tests
replace the regex pins; behavior identical.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/collaboration/collaboration.handler.ts

@@ -9,10 +9,8 @@ import { setYjsMark, updateYjsMarkAttribute, YjsSelection } from './yjs.util';
 import * as Y from 'yjs';
 import { User } from '@docmost/db/types/entity.types';
 import {
-  hasHtmlEmbedNode,
-  htmlEmbedAllowed,
   isHtmlEmbedFeatureEnabled,
-  stripHtmlEmbedNodes,
+  stripHtmlEmbedIfNotAllowed,
 } from '../common/helpers/prosemirror/html-embed.util';
 import { WorkspaceRepo } from '@docmost/db/repos/workspace/workspace.repo';
 
@@ -106,14 +104,14 @@ export class CollaborationHandler {
         const htmlEmbedEnabled = isHtmlEmbedFeatureEnabled(
           (await this.workspaceRepo.findById(user?.workspaceId))?.settings,
         );
-        if (!htmlEmbedAllowed(htmlEmbedEnabled, user?.role)) {
-          if (hasHtmlEmbedNode(prosemirrorJson)) {
+        prosemirrorJson = stripHtmlEmbedIfNotAllowed(prosemirrorJson, {
+          featureEnabled: htmlEmbedEnabled,
+          role: user?.role,
+          onStrip: () =>
             this.logger.warn(
               `Stripping htmlEmbed node(s) from update by user ${user?.id} on ${documentName}`,
-            );
-            prosemirrorJson = stripHtmlEmbedNodes(prosemirrorJson);
-          }
-        }
+            ),
+        });
 
         await this.withYdocConnection(
           hocuspocus,