langchain-openrouter: ToolMessage content blocks (e.g. images) are sent to the API unconverted, failing client-side with ValidationError

### 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 - [ ] 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 - [x] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs https://github.com/langchain-ai/langchain/issues/34328 https://github.com/langchain-ai/langchain/issues/31950 ### Reproduction Steps / Example Code (Python) ```python ChatOpenRouter converts standard LangChain content blocks only for user messages. A tool result containing a standard image content block — exactly what file-reading tools (e.g. deepagents' read_file) return for a PNG — is forwarded to the OpenRouter API unconverted, and the openrouter SDK rejects it client-side before any request is sent. The script runs fully offline — no valid API key needed, because the failure happens during request validation. The text-only tool result (control) passes validation and reaches the network (it fails there with 401 because of the dummy key); the image tool result never gets that far. import base64 from langchain_core.message…