fix: unmount editor in ForkYDoc tests to prevent unhandled errors

ProseMirror's DOMObserver timeout fires after jsdom teardown, causing
"document is not defined" ReferenceError. Adding editor.unmount() at
the end of each test ensures proper cleanup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nperez0111

packages/core/src/extensions/Collaboration/ForkYDoc.test.ts

@@ -52,6 +52,8 @@ it("can fork a document", async () => {
   await expect(editor.document).toMatchFileSnapshot(
     "__snapshots__/fork-yjs-snap-editor-forked.json",
   );
+
+  editor.unmount();
 });
 
 it("can merge a document", async () => {
@@ -108,6 +110,8 @@ it("can merge a document", async () => {
   await expect(editor.document).toMatchFileSnapshot(
     "__snapshots__/fork-yjs-snap-editor.json",
   );
+
+  editor.unmount();
 });
 
 it("can fork an keep the changes to the original document", async () => {
@@ -164,4 +168,6 @@ it("can fork an keep the changes to the original document", async () => {
   await expect(editor.document).toMatchFileSnapshot(
     "__snapshots__/fork-yjs-snap-editor-forked.json",
   );
+
+  editor.unmount();
 });