test: restore the four workspace-merge tests broken by a stale mock

project-engine.test.ts hand-writes a factory for ../merge/merger-ai.js, and
it had drifted from the module: WorkspaceMergeDispatchSupersededError was
missing. Production imports that class, so the merge-queue drain threw "No
<export> is defined on the mock" before the code under test ran — the four
Phase C hardening cases then failed on a resolved promise and an absent
updateTask call, for a reason unrelated to what they assert.

No product change. All 114 tests in the file pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.changeset/workspace-merge-test-mock-export.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Repair four red workspace-merge tests caused by a stale module mock.
+category: internal
+dev: `project-engine.test.ts` mocks `../merge/merger-ai.js` with a hand-written factory that had not kept up with the module's exports: `WorkspaceMergeDispatchSupersededError` was missing. Production imports it, so the merge-queue drain threw "No <export> is defined on the mock" before reaching the behaviour under test, and the four Phase C hardening cases failed on a resolved promise and a missing `updateTask` call rather than on what they assert. No product change; the factory now provides the class and carries a note to keep it in step with merger-ai's exported errors.

packages/engine/src/__tests__/project-engine.test.ts

@@ -136,12 +136,26 @@ vi.mock("../merge/merger-ai.js", () => {
       this.name = "WorkspaceFinalizeBlockedError";
     }
   }
+  /*
+  FNXC:WorkspaceMerge 2026-08-19-04:00:
+  Production imports this error from merger-ai, so a factory that omits it makes the merge-queue
+  drain throw "No <export> is defined on the mock" BEFORE reaching the behaviour under test — the
+  four Phase C hardening cases then saw a resolved promise and no updateTask call, failing for a
+  reason unrelated to what they assert. Keep this list in step with merger-ai's exported errors.
+  */
+  class WorkspaceMergeDispatchSupersededError extends Error {
+    constructor(public readonly taskId: string) {
+      super(`Workspace merge dispatch lease was superseded before finalization for ${taskId}`);
+      this.name = "WorkspaceMergeDispatchSupersededError";
+    }
+  }
   return {
     runAiMerge: mocks.runAiMerge,
     landWorkspaceTask: mocks.landWorkspaceTask,
     WorkspaceRepoLandBusyError,
     WorkspacePartialLandError,
     WorkspaceFinalizeBlockedError,
+    WorkspaceMergeDispatchSupersededError,
   };
 });