core[patch]: Fix #5873 (LCEL issue when streaming from LLMs) (#5874)

* Fix #5873 (LCEL issue when streaming from LLMs)

Fix #5873.

* Add test

---------

Co-authored-by: jacoblee93 <jacoblee93@gmail.com>

langchain-core/src/language_models/llms.ts

@@ -156,7 +156,7 @@ export abstract class BaseLLM<
       });
       try {
         for await (const chunk of this._streamResponseChunks(
-          input.toString(),
+          prompt.toString(),
           callOptions,
           runManagers?.[0]
         )) {

langchain-core/src/language_models/tests/llms.test.ts

@@ -1,7 +1,8 @@
 /* eslint-disable no-promise-executor-return */
 
 import { test } from "@jest/globals";
-import { FakeLLM } from "../../utils/testing/index.js";
+import { FakeLLM, FakeStreamingLLM } from "../../utils/testing/index.js";
+import { HumanMessagePromptTemplate } from "../../prompts/chat.js";
 
 test("Test FakeLLM uses callbacks", async () => {
   const model = new FakeLLM({});
@@ -40,3 +41,16 @@ test("Test FakeLLM uses callbacks with a cache", async () => {
   expect(response).toEqual(response2);
   expect(response2).toEqual(acc);
 });
+
+test("Test FakeStreamingLLM works when streaming through a prompt", async () => {
+  const prompt = HumanMessagePromptTemplate.fromTemplate("hello there {name}");
+  const model = new FakeStreamingLLM({});
+  const chain = prompt.pipe(model);
+  const stream = await chain.stream({ name: "test" });
+  const chunks = [];
+  for await (const chunk of stream) {
+    chunks.push(chunk);
+  }
+  expect(chunks.length).toBeGreaterThan(1);
+  expect(chunks.join("")).toEqual("Human: hello there test");
+});