FN-5796: make plugin same-state transitions idempotent

Prevent duplicate-state plugin updates from throwing invalid transition errors.

- treat updatePluginState calls where next state equals current state as idempotent no-ops
- allow same-state updates with an explicit error payload to persist error changes and emit plugin:updated
- add regression tests for same-state no-op behavior, no extra stateChanged event emission, and error payload updates
- add a patch changeset documenting the Dependency Graph plugin fix

Files changed:
 .changeset/fn-5796-plugin-state-noop.md          |  7 +++++
 packages/core/src/__tests__/plugin-store.test.ts | 34 ++++++++++++++++++++++++
 packages/core/src/plugin-store.ts                | 15 +++++++++++
 3 files changed, 56 insertions(+)

Fusion-Task-Id: FN-5796

Fusion-Task-Lineage: 0da1d035-383f-456c-a212-0903dc029874

.changeset/fn-5796-plugin-state-noop.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix a spurious Settings → Plugins error for the bundled Dependency Graph plugin where plugin startup could fail with `Invalid state transition from "started" to "started"`.
+
+Plugin state transitions now treat same-state updates as idempotent no-ops, while still allowing same-state calls with an explicit error payload to update the persisted error field without emitting a state-changed transition.