feat(langchain): add toolEmulator middleware

[christian-bromann]

Ports the LLMToolEmulator middleware from Python to TypeScript, enabling tool emulation for testing purposes. The middleware allows selective emulation of tool calls using an LLM instead of executing actual tool implementations.

Features

• Emulate all tools (default) or specific tools by name/instance
• Configurable model for emulation (defaults to anthropic:claude-sonnet-4-5-20250929)
• Type-safe TypeScript implementation following existing middleware patterns

Usage

import { toolEmulatorMiddleware } from "@langchain/langchain/agents/middleware";

// Emulate all tools
const middleware = toolEmulatorMiddleware();

// Emulate specific tools
const middleware = toolEmulatorMiddleware({
  tools: ["get_weather", "get_user_location"],
  model: "anthropic:claude-sonnet-4-5-20250929"
});

const agent = createAgent({
  model: "openai:gpt-4o",
  tools: [getWeather, getUserLocation],
  middleware: [middleware],
});

Changes

• Added toolEmulatorMiddleware implementation (libs/langchain/src/agents/middleware/toolEmulator.ts)
• Added comprehensive test suite with provider mocks
• Exported middleware from middleware/index.ts

Testing

Full test coverage including initialization, tool filtering, model configuration, and integration with createAgent.

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