feat(anthropic): support websearch tool

[christian-bromann]

This PR adds support for Anthropic's Web Search tool (web_search_20250305) to @langchain/anthropic, enabling Claude to perform real-time web searches and cite sources in its responses.

Usage

import { ChatAnthropic, tools } from "@langchain/anthropic";

const model = new ChatAnthropic({ model: "claude-sonnet-4-5-20250929" });
const modelWithSearch = model.bindTools([tools.webSearch_20250305({ maxUses: 3 })]);

const response = await modelWithSearch.invoke("What's the latest news about AI?");

Based on the direction from #9406 this would implement the websearch tool. Here is the example output from the example script:

{
  messages: [
    HumanMessage {
      "id": "c7416f2c-bea0-4e49-a3b6-384b23099211",
      "content": "What's the weather in NYC?",
      "additional_kwargs": {},
      "response_metadata": {}
    },
    AIMessage {
      "id": "msg_01CqUm4rR4KrgtUvemNA1gfK",
      "content": [
        {
          "type": "server_tool_use",
          "id": "srvtoolu_01NottiD8nsS9Y4H64A1TEsb",
          "name": "web_search",
          "input": {
            "query": "NYC weather today"
          }
        },
        {
          "type": "web_search_tool_result",
          "tool_use_id": "srvtoolu_01NottiD8nsS9Y4H64A1TEsb",
          "content": [
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]",
            "[Object]"
          ]
        },
        {
          "type": "text",
          "text": "Based on the latest weather information for New York City:\n\n**Today (Thursday, November 14):**\n"
        },
        {
          "citations": [
            "[Object]"
          ],
          "type": "text",
          "text": "Partly cloudy skies in the morning will give way to cloudy skies during the afternoon, with a high near 53°F and winds from the west-northwest at 10 to 15 mph."
        },
        {
          "type": "text",
          "text": "\n\n**Tonight:**\n"
        },
        {
          "citations": [
            "[Object]"
          ],
          "type": "text",
          "text": "Mostly cloudy, then gradually becoming mostly clear, with a low around 42°F and west wind around 13 mph, with gusts as high as 24 mph."
        },
        {
          "type": "text",
          "text": "\n\n**Recent Weather Note:**\n"
        },
        {
          "citations": [
            "[Object]"
          ],
          "type": "text",
          "text": "NYC saw its first snow of the season on Tuesday, with wintry showers during the morning commute."
        },
        {
          "type": "text",
          "text": "\n\nThe weather is cool and breezy today with temperatures in the low 50s, typical for mid-November in New York City."
        }
      ],
      "name": "model",
      "additional_kwargs": {
        "model": "claude-sonnet-4-5-20250929",
        "id": "msg_01CqUm4rR4KrgtUvemNA1gfK",
        "type": "message",
        "role": "assistant",
        "stop_reason": "end_turn",
        "stop_sequence": null,
        "usage": {
          "input_tokens": 9422,
          "cache_creation_input_tokens": 0,
          "cache_read_input_tokens": 0,
          "cache_creation": {
            "ephemeral_5m_input_tokens": 0,
            "ephemeral_1h_input_tokens": 0
          },
          "output_tokens": 290,
          "service_tier": "standard",
          "server_tool_use": {
            "web_search_requests": 1
          }
        }
      },
      "response_metadata": {
        "model": "claude-sonnet-4-5-20250929",
        "id": "msg_01CqUm4rR4KrgtUvemNA1gfK",
        "stop_reason": "end_turn",
        "stop_sequence": null,
        "usage": {
          "input_tokens": 9422,
          "cache_creation_input_tokens": 0,
          "cache_read_input_tokens": 0,
          "cache_creation": {
            "ephemeral_5m_input_tokens": 0,
            "ephemeral_1h_input_tokens": 0
          },
          "output_tokens": 290,
          "service_tier": "standard",
          "server_tool_use": {
            "web_search_requests": 1
          }
        },
        "type": "message",
        "role": "assistant",
        "model_provider": "anthropic"
      },
      "tool_calls": [],
      "invalid_tool_calls": [],
      "usage_metadata": {
        "input_tokens": 9422,
        "output_tokens": 290,
        "total_tokens": 9712,
        "input_token_details": {
          "cache_creation": 0,
          "cache_read": 0
        }
      }
    }
  ]
}

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