FN-7004: expect merge abort errors in dependency install test

Align the dependency-install abort regression test with the merge layer's canonical abort error.

- Update the dependency-sync abort expectation from AbortError to MergeAbortedError.
- Document why landOneRepo normalizes inner AbortError values through throwIfAborted.

Files changed:
 .../engine/src/__tests__/merger-ai-dependency-install.slow.test.ts  | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-7004

Fusion-Task-Lineage: 7b8a515a-cbbf-4e57-91cd-05a9a1001fd5

packages/engine/src/__tests__/merger-ai-dependency-install.slow.test.ts

@@ -233,10 +233,14 @@ describe("runAiMerge dependency install", () => {
     abortStore.appendAgentLog.mockImplementation(async (_id: string, message: string) => {
       if (String(message).includes("Syncing dependencies")) controller.abort();
     });
+    /*
+    FNXC:AIMerge 2026-06-25-04:07:
+    Merge-layer aborts must surface as the canonical MergeAbortedError consumed by project-engine.ts abort detection; landOneRepo intentionally re-normalizes dependency-sync inner AbortError via throwIfAborted.
+    */
     await expect(runAiMerge(abortStore, abortDir, "FN-1", { manual: true, signal: controller.signal }, {
       mergeAgent: realMergeAgent(),
       reviewAgent: vi.fn(async () => "REVIEW_VERDICT: approve"),
-    })).rejects.toMatchObject({ name: "AbortError" });
+    })).rejects.toMatchObject({ name: "MergeAbortedError" });
   });
 
   it("runs dependency install again after a concurrent integration advance rebuild", async () => {