C4 follow-up #3: RFC 8594 Deprecation/Sunset headers + call counter on legacy hire routes

tests/unit/deprecation-counter.test.ts

@@ -0,0 +1,43 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import {
+  incrementDeprecationCall,
+  readDeprecationCounters,
+  resetDeprecationCounters,
+} from "../../src/main/api-server/deprecation-counter";
+
+/**
+ * C4-followup #3: the in-process deprecation counter backs the
+ * "has legacy traffic hit zero?" decision. Keep these tests tight —
+ * the module has one job.
+ */
+describe("deprecation counter", () => {
+  beforeEach(() => {
+    resetDeprecationCounters();
+  });
+
+  it("increments and reads back the count for a single route", () => {
+    incrementDeprecationCall("POST /legacy/hire");
+    incrementDeprecationCall("POST /legacy/hire");
+    expect(readDeprecationCounters()).toEqual({ "POST /legacy/hire": 2 });
+  });
+
+  it("tracks multiple route labels independently", () => {
+    incrementDeprecationCall("POST /legacy/hire");
+    incrementDeprecationCall("POST /legacy/approvals");
+    incrementDeprecationCall("POST /legacy/hire");
+    expect(readDeprecationCounters()).toEqual({
+      "POST /legacy/hire": 2,
+      "POST /legacy/approvals": 1,
+    });
+  });
+
+  it("resets all counters back to empty", () => {
+    incrementDeprecationCall("POST /legacy/hire");
+    resetDeprecationCounters();
+    expect(readDeprecationCounters()).toEqual({});
+  });
+
+  it("throws on an empty routeLabel instead of silently recording", () => {
+    expect(() => incrementDeprecationCall("")).toThrow(/non-empty routeLabel/);
+  });
+});