Duplicate tool_calls in content_blocks with OpenAI Responses API streaming

### 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 - [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 Not exact duplicates, but potentially related: #34406, #34323 ### Reproduction Steps / Example Code (Python) ```python """ Minimal reproduction: content_blocks includes duplicate tool_calls when using OpenAI Responses API streaming. No API key needed - demonstrates the core issue. """ from langchain_core.messages import AIMessage, ToolMessage def main(): # Simulates what OpenAI Responses API streaming produces: # - content: accumulates fc_ prefixed blocks (partial, empty args) # - tool_calls: final call_ prefixed blocks (full args) simulated_content = [ {"type": "text", "text": "Let me check the weather."}, # fc_ blocks accumulated during streaming (partial, no args) {"type": "tool_call", "id": "fc_12345", "name": "get_weather", "args": {}}, {"type": "tool_call", "id": "f…