fix: refuse a move while an editor holds text that is not valid JSON

That text never reaches the draft, and the move closes the editor holding
it. The move now refuses before closing anything, as opening a session
from a page drawer already does.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/sessions/moveSessionToFork.ts

@@ -5,7 +5,10 @@ import { UserDraftDbSyncer } from '$lib/userDraftDbSyncer.svelte'
 import { invalidateWorkspaceDrafts } from '$lib/workspaceDrafts.svelte'
 import { invalidateWorkspaceDiffCache } from '$lib/components/copilot/chat/global/diffSnapshot'
 import { logFeatureUsage } from '$lib/utils/featureUsage'
-import { flushAllPendingEditorChanges } from '$lib/components/pendingEditorFlush'
+import {
+	anyEditorUnparseable,
+	flushAllPendingEditorChanges
+} from '$lib/components/pendingEditorFlush'
 import { flushEditorSaves } from './useUserDraftSync.svelte'
 import {
 	materializeForkOutcome,
@@ -44,6 +47,10 @@ export type MoveSessionResult =
  */
 export async function flushPendingSaves(workspace: string): Promise<string | undefined> {
 	flushAllPendingEditorChanges()
+	// Text that does not parse never reaches the draft, and the move closes the editor holding it.
+	if (anyEditorUnparseable()) {
+		return 'An editor has changes that are not valid JSON. Fix them before moving.'
+	}
 	await tick()
 	flushEditorSaves(workspace)
 	await tick()