feat: harden workflow gates and expose token usage

backend/test/session-bridge.test.ts

@@ -42,6 +42,43 @@ test("real Pi thinking_delta becomes a dedicated activity_delta to the FE", () =
   expect(seen).toEqual([{ type: "activity_delta", text: "Valuto le ambiguità" }]);
 });
 
+test("assistant message_end exposes sanitized token usage with the configured context window", () => {
+  const { rpc, fire } = fakeRpc();
+  const bridge = new SessionBridge(rpc);
+  const seen: any[] = [];
+  bridge.onClientEvent((event) => seen.push(event));
+  bridge.setContextWindow(200_000);
+
+  fire({
+    type: "message_end",
+    message: {
+      role: "assistant",
+      stopReason: "stop",
+      usage: {
+        input: 110_348,
+        cacheRead: 633_344,
+        cacheWrite: 12,
+        output: 33_652,
+        totalTokens: 777_356,
+        cost: { total: 99 },
+      },
+      content: "DO_NOT_FORWARD",
+    },
+  });
+
+  expect(seen).toEqual([{
+    type: "usage",
+    usage: {
+      input: 110_348,
+      cacheRead: 633_344,
+      output: 33_652,
+      totalTokens: 777_356,
+      contextWindow: 200_000,
+    },
+  }]);
+  expect(JSON.stringify(seen)).not.toMatch(/DO_NOT_FORWARD|cost|cacheWrite/);
+});
+
 test("assistant provider errors are sanitized and leave the turn failed", () => {
   const { rpc, fire } = fakeRpc();
   const bridge = new SessionBridge(rpc);