Add support for Anthropic via Bedrock's `tool_search_tool_regex` in `ChatAnthropicBedrock`

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [x] langchain - [ ] langchain-openai - [x] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description **Package:** `langchain-anthropic` v1.4.0 ## Description When using Anthropic's tool search feature (`tool_search_tool_regex`) via Bedrock, the model returns `tool_search_tool_result` content blocks in its assistant messages. On subsequent turns, `langchain-anthropic` serializes these blocks back into the messages payload with an extra `index` field that the Bedrock API rejects: ``` anthropic.BadRequestError: Error code: 400 - {'message': 'messages.7.content.2.tool_search_tool_result.index: Extra inputs are not permitted'} ``` Attempting to use the anthropic tool defined here: https://docs.aws.amazon.com/bedrock/latest/userguide/model-parameters-anthropic-claude-messages-tool-use.html#model-parameters-anthropic-claude-tool-search-tool ## Reproduction 1. Configure `ChatAnthropicBedrock` with the `tool-search-tool-2025-10-19` beta and a `tool_search_tool_regex` entry in the tools array 2. Mark at least one tool with `defer_loading: True` (via `extras={"defer_loading": True}` …