feat(FN-3324): add runtime guard regression sentinel

Adds a regression test sentinel in the engine's in-process runtime test suite to guard against runtime guard behavior regressions (FN-3324).

Fusion-Task-Id: FN-3324

packages/engine/src/__tests__/in-process-runtime.test.ts

@@ -0,0 +1,11 @@
+import { describe, it, expect } from "vitest";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+describe("InProcessRuntime onStart duplicate guard", () => {
+  it("contains a taskAgentMap guard before creating task-worker agents", () => {
+    const source = readFileSync(join(process.cwd(), "src/runtimes/in-process-runtime.ts"), "utf-8");
+    expect(source).toContain("if (this.taskAgentMap.has(task.id))");
+    expect(source).toContain("Skipping task-worker creation for");
+  });
+});