add proxy cookie leak repro test

tests/routes/proxy.test.ts

@@ -72,4 +72,37 @@ describe("proxy route", () => {
       mockServer.stop()
     }
   })
+
+  test.failing(
+    "should not forward proxy-origin cookies to the target",
+    async () => {
+      const { axios } = await getTestServer()
+
+      const mockServerPort = 4001
+      const mockServer = Bun.serve({
+        port: mockServerPort,
+        fetch(req) {
+          return Response.json({
+            cookie: req.headers.get("cookie"),
+          })
+        },
+      })
+
+      try {
+        const response = await axios.get("/proxy", {
+          headers: {
+            "X-Target-Url": `http://localhost:${mockServerPort}`,
+            "X-Sender-Cookie": "",
+            Cookie:
+              'ph_test_posthog={"distinct_id":"localhost-user","session_id":"localhost-session"}',
+          },
+        })
+
+        expect(response.status).toBe(200)
+        expect(response.data.cookie).toBeNull()
+      } finally {
+        mockServer.stop()
+      }
+    },
+  )
 })