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

docs/dashboard-guide.md

@@ -113,7 +113,7 @@ Planning Mode now includes branch controls on the summary screen before you crea
 
 These values are sent with the Planning Mode create-task request as `branchSelection`, so created tasks persist branch/base-branch settings consistently with other branch-aware task creation flows.
 
-Completed single-task planning sessions remain in the Planning Mode history after you create the task, and selecting one restores the completed summary instead of restarting the composer.
+Completed single-task planning sessions remain in the Planning Mode history after you create the task, and selecting one restores the completed summary instead of restarting the composer. History rows are deduplicated by session id even if the initial load and live session updates arrive out of order, and deleting a history entry now waits for the server delete to persist (failures keep the row visible and surface an error instead of silently disappearing until refresh).
 
 ## New Task Modal Branch Strategy