test(middleware): replace integration test with unit test using FakeListChatModel

Replace the llmToolSelector integration test that required a real OpenAI API
call with a unit test using FakeListChatModel, so the streaming isolation
check runs without network access or API keys.

Author: JadenKim-dev

libs/langchain/src/agents/middleware/tests/llmToolSelector.int.test.ts

@@ -10,6 +10,7 @@ import {
 import { tool } from "@langchain/core/tools";
 import { HumanMessage, AIMessage } from "@langchain/core/messages";
 import { BaseChatModel } from "@langchain/core/language_models/chat_models";
+import { FakeListChatModel } from "@langchain/core/utils/testing";
 
 import { llmToolSelectorMiddleware } from "../llmToolSelector.js";
 import { createAgent } from "../../index.js";
@@ -417,3 +418,83 @@ describe("llmToolSelectorMiddleware", () => {
     expect(firstCall[1].content).toContain("Second message");
   });
 });
+
+describe("llmToolSelectorMiddleware – streaming isolation", () => {
+  const getWeather = tool(
+    ({ location }: { location: string }) =>
+      `Weather in ${location}: Sunny, 72°F`,
+    {
+      name: "get_weather",
+      description: "Get current weather for a location",
+      schema: z.object({
+        location: z.string().describe("City name"),
+      }),
+    }
+  );
+
+  const searchDatabase = tool(
+    ({ customerId }: { customerId: string }) =>
+      `Customer ${customerId}: Premium account`,
+    {
+      name: "search_database",
+      description: "Look up customer information by customer ID",
+      schema: z.object({
+        customerId: z.string().describe("Customer ID"),
+      }),
+    }
+  );
+
+  const calculatePrice = tool(
+    ({ items, discount }: { items: number; discount: number }) =>
+      `Total: $${(items * 29.99 * (1 - discount / 100)).toFixed(2)}`,
+    {
+      name: "calculate_price",
+      description: "Calculate pricing with discounts",
+      schema: z.object({
+        items: z.number(),
+        discount: z.number(),
+      }),
+    }
+  );
+
+  it("does not leak tool-selector output into messages stream", async () => {
+    // given
+    const selectorModel = new FakeListChatModel({
+      responses: [JSON.stringify({ tools: ["get_weather"] })],
+    });
+    const agentModel = new FakeListChatModel({
+      responses: ["The weather in Seoul is sunny and 72°F."],
+    });
+
+    const middleware = llmToolSelectorMiddleware({
+      model: selectorModel,
+      maxTools: 1,
+    });
+
+    const agent = createAgent({
+      model: agentModel,
+      tools: [getWeather, searchDatabase, calculatePrice],
+      middleware: [middleware],
+    });
+
+    // when
+    const stream = await agent.stream(
+      { messages: [new HumanMessage("What's the weather in Seoul?")] },
+      { streamMode: "messages" }
+    );
+
+    const parts: string[] = [];
+    for await (const chunk of stream) {
+      parts.push(JSON.stringify(chunk));
+    }
+    const serialized = parts.join("");
+
+    // then
+    const hasToolSelectorLeak =
+      serialized.includes('"content":"{\\"tools') ||
+      serialized.includes('"content":"tools') ||
+      /"content":"[^"]*tools[^"]*","tool_call_chunks":\[\]/.test(serialized);
+
+    expect(hasToolSelectorLeak).toBe(false);
+  });
+});