feat(chat): handle credit-gated send responses

src/stores/chat/__tests__/chat-machine.transitions.test.ts

@@ -467,6 +467,11 @@ describe("chatMachine transitions", () => {
               limit: 3,
             },
           },
+          canSendMessage: false,
+          cannotSendReason: "insufficient_credits",
+          creditBalance: 0,
+          requiredCredits: 2,
+          shortfallCredits: 2,
         }),
         reply: null,
       },
@@ -474,11 +479,14 @@ describe("chatMachine transitions", () => {
 
     expect(actor.getSnapshot().context.upgradePromptVisible).toBe(true);
     expect(actor.getSnapshot().context.upgradeReason).toBe("weekly_limit");
+    expect(actor.getSnapshot().context.canSendMessage).toBe(false);
 
     actor.send({ type: "ChatPaymentSucceeded" });
 
     expect(actor.getSnapshot().context.upgradePromptVisible).toBe(false);
     expect(actor.getSnapshot().context.upgradeReason).toBeNull();
+    expect(actor.getSnapshot().context.canSendMessage).toBe(true);
+    expect(actor.getSnapshot().context.cannotSendReason).toBeNull();
 
     actor.stop();
   });