FN-5892: dedupe planning sessions and persist history deletion

Prevent duplicate planning-history rows and keep deletions visible only after the backend confirms persistence.

- dedupe Planning Mode history by session id across initial loads, live updates, archive toggles, and draft creation
- make deleteAiSession surface backend/content-type errors instead of silently succeeding on failed deletes
- keep failed history deletions visible, refresh the list, and show an error toast when persistence fails
- add dashboard tests covering session deduplication, delete API failures, and persistence-aware Planning Mode deletion behavior
- document the history dedupe/delete semantics and add a patch changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5892-planning-history-fix.md         |   5 +
 docs/dashboard-guide.md                            |   2 +-
 packages/dashboard/app/api/__tests__/deleteAiSession.test.ts      |  54 +++++
 packages/dashboard/app/api/legacy.ts               |  41 +++-
 packages/dashboard/app/components/PlanningModeModal.tsx |  70 ++++---
 packages/dashboard/app/components/__tests__/PlanningModeModal.autosize.test.tsx  |  11 +
 packages/dashboard/app/components/__tests__/PlanningModeModal.dedupeSessionsById.test.ts   |  44 ++++
 packages/dashboard/app/components/__tests__/PlanningModeModal.initial.test.tsx   |  11 +
 packages/dashboard/app/components/__tests__/PlanningModeModal.planning-flow.test.tsx       | 226 ++++++++++++++++++++-
 packages/dashboard/app/components/__tests__/PlanningModeModal.test-helpers.ts    |   1 +
 10 files changed, 439 insertions(+), 26 deletions(-)

Fusion-Task-Id: FN-5892

Fusion-Task-Lineage: 7453bc34-6d57-4d67-ac92-8fb50084fbee

.changeset/fn-5892-planning-history-fix.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix Planning Mode session history so duplicate AI-session rows are collapsed by session id and deleting a history entry only succeeds when the server-side delete persists.