Empty `AIMessage` when using `astream()` with Anthropic programmatic tool calling

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [x] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Example Code (Python) ```python """ Minimal reproduction of langchain-anthropic streaming bug with code execution. Bug: When streaming with Anthropic code execution that calls external tools, the `message_start` event can contain pre-completed content blocks, but `_make_message_chunk_from_anthropic_event` ignores this content and creates an empty chunk. This results in an empty AIMessage after aggregation. To run: pip install langchain-anthropic export ANTHROPIC_API_KEY=your_key python minimal_repro_streaming_bug.py """ import asyncio import os from langchain_anthropic import ChatAnthropic from langchain_core.messages import HumanMessage, ToolMessage from langchain_core.tools import tool @tool(extras={"allowed_callers": ["code_execution_20250825"]}) def calculate(expression: str) -> str: """Evaluate a mathema…