test(coverage): add regression tests for issues #192, #206, #204

Additive test coverage across server, editor-ext, client and mcp.

#192 — AiChatService.stream integration (Section 3, against real Postgres):
- new apps/server/test/integration/ai-chat-stream.int-spec.ts drives the real
  streamText through a seeded ai/test MockLanguageModelV3 and a real Node
  ServerResponse, covering: onError persists an assistant error record
  (status 'error' + partial answer + provider cause in metadata); external MCP
  client closed exactly once on BOTH onFinish and onError; anti-tamper —
  history is rebuilt from the DB transcript, not from body.messages.

#206 — red-team findings (most already fixed+tested in #212):
- mdrt-2 (UNFIXED, data loss): turndown.dataloss.test.ts documents that
  pageBreak / transclusionReference / mention are silently dropped on Markdown
  export (characterization + it.fails for the desired survive-export contract).
- persist-6 (UNFIXED, data loss): persistence-store.spec.ts adds an it.failing
  documenting that a momentarily-empty live doc overwrites non-empty content
  (left unfixed — a store-side empty-guard is a behaviour change).

#204 — test-strategy plan, highest-priority subset:
- Phase 1: mcp-clients.lease.spec.ts covers the external MCP client
  lease/refcount/eviction lifecycle (leak / premature-close / double-close).
- Phase 2 data-integrity pure functions: editor-ext table-utils
  (transpose/moveRow/convert round-trip) and math tokenizer false-positive
  guard; client emoji-menu (+ it.fails for the unguarded localStorage
  JSON.parse bug), sort-cells, normalizeTableColumnWidths; mcp htmlEmbed/
  pageBreak markdown data-loss + footnote-diff; server export
  getInternalLinkPageName extensionless-path bug — FIXED (small/clear) + tested.

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

apps/server/src/collaboration/extensions/persistence-store.spec.ts

@@ -205,6 +205,32 @@ describe('PersistenceExtension.onStoreDocument — Approach-A boundary snapshot'
     expect(historyQueue.add).toHaveBeenCalledTimes(1);
   });
 
+  // #206 persist-6 — RED (it.failing): a momentarily-empty live Y.Doc must not
+  // overwrite non-empty persisted content. `onStoreDocument` empty-guards the
+  // LOAD path but not the STORE path, so today an empty doc (a client/agent
+  // glitch, a bad merge, an emptying transclusion) is written straight over the
+  // page and the content is wiped silently. A store-side empty-guard is a real
+  // behaviour change (a deliberate "select-all + delete" is also empty), so it
+  // is left UNFIXED pending a product decision; this documents the data-loss
+  // path and flips to a normal passing test the moment the guard lands.
+  it.failing(
+    'does NOT overwrite non-empty content with a momentarily-empty live doc (persist-6)',
+    async () => {
+      const emptyDoc = { type: 'doc', content: [{ type: 'paragraph' }] };
+      const document = ydocFor(emptyDoc);
+      pageRepo.findById.mockResolvedValue({
+        ...persistedHumanPage('IGNORED'),
+        content: doc('IMPORTANT RICH CONTENT'),
+      });
+
+      await ext.onStoreDocument(buildData(document, 'user') as any);
+
+      // Desired contract: the empty incoming doc is rejected and the rich page
+      // survives. Today updatePage is called with the empty content (data loss).
+      expect(pageRepo.updatePage).not.toHaveBeenCalled();
+    },
+  );
+
   // persist-1 — when every attempt fails the hook must NOT report a phantom
   // success: no "page.updated" badge broadcast and no history snapshot for
   // content that was never written.