feat(FN-3263): strengthen worker-budget regression test coverage

Strengthens regression test coverage for worker-budget behavior in the vitest workers test suite, adding 14 lines to cover edge cases.

Fusion-Task-Id: FN-3263

packages/core/src/__tests__/vitest-workers.test.ts

@@ -0,0 +1,56 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { cpus } from "node:os";
+import { computeMaxWorkers } from "../__test-utils__/vitest-workers";
+
+const ORIGINAL_ENV = { ...process.env };
+
+describe("computeMaxWorkers", () => {
+  afterEach(() => {
+    process.env = { ...ORIGINAL_ENV };
+  });
+
+  it("uses explicit VITEST_MAX_WORKERS for package-scoped runs", () => {
+    process.env.VITEST_MAX_WORKERS = "4";
+    delete process.env.FUSION_TEST_TOTAL_WORKERS;
+    delete process.env.FUSION_TEST_CONCURRENCY;
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+    const cpuCap = Math.max(1, cpus().length - 1);
+
+    expect(workers).toBe(Math.min(4, cpuCap));
+    expect(process.env.VITEST_MAX_WORKERS).toBe(String(workers));
+  });
+
+  it("clamps explicit VITEST_MAX_WORKERS to workspace per-package budget", () => {
+    process.env.VITEST_MAX_WORKERS = "4";
+    process.env.FUSION_TEST_TOTAL_WORKERS = "4";
+    process.env.FUSION_TEST_CONCURRENCY = "2";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(2);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("2");
+  });
+
+  it("still derives workers from workspace budget when explicit override is absent", () => {
+    delete process.env.VITEST_MAX_WORKERS;
+    process.env.FUSION_TEST_TOTAL_WORKERS = "6";
+    process.env.FUSION_TEST_CONCURRENCY = "2";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(3);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("3");
+  });
+
+  it("ignores invalid env values and falls back to default cap", () => {
+    process.env.VITEST_MAX_WORKERS = "abc";
+    process.env.FUSION_TEST_TOTAL_WORKERS = "0";
+    process.env.FUSION_TEST_CONCURRENCY = "-1";
+
+    const workers = computeMaxWorkers({ defaultCap: 2 });
+
+    expect(workers).toBe(2);
+    expect(process.env.VITEST_MAX_WORKERS).toBe("2");
+  });
+});