core: `_convert_to_v1_from_anthropic` mutates message.content when lifting `index` off an unrecognized block

### Submission checklist - [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 - [x] 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 ### Related Issues / PRs Same broader defect class raised in #40001 ("AIMessage.content_blocks mutates message.content in the Anthropic, Bedrock and Google translators"), which explicitly identified this exact Anthropic-translator case. #40001 was closed as completed after #40022 (bedrock_converse fix) and #40023 (google-genai fix) merged, but neither of those touches anthropic.py -- this specific half was never fixed. bedrock.py's _convert_to_v1_from_bedrock() imports and calls _convert_to_v1_from_anthropic() directly, so model_provider="bedrock" (for Claude-backed models) inherits the same bug. ### Reproduction Steps / Example Code (Python) ```python from copy import deepcopy from langchain_core.messages import AIMessage message = AIMessage( content=[{"type": "custom", "payload": "value", "index": 3}], response_metadata={"model_provider":…