openai: malformed tool calls cause all tool calls to be silently dropped

### 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 - [x] langchain-openai ### Reproduction Steps / Example Code (Python) ```python #!/usr/bin/env python3 """ Minimal reproduction for: malformed tool calls cause all tool calls to be silently dropped This script demonstrates a bug in _convert_delta_to_message_chunk where a malformed tool call (missing 'function' key) causes ALL tool calls in the batch to be silently dropped. """ from langchain_openai.chat_models.base import _convert_delta_to_message_chunk # Test case: One malformed tool call in a batch causes ALL to be dropped mixed_delta = { "role": "assistant", "content": None, "tool_calls": [ { "id": "call_good", "type": "function", "function": {"name": "good_func", "arguments": "{}"}, "index": 0 }, { "id": "call_bad", "type": "function", # Missing 'function' key - causes KeyError "index": 1 } ] } result = _convert_delta_to_message_chunk(mixed_delta, None) print(f"Tool call chunks count: {len(result.tool_call_chunks)}") # Expected: 1 valid tool call chunk (the good one) # Actual: 0 tool call chunks (entire batch lost!) ``` ### Error Message and Stack Trace ``` No exception is raised - this is a silent data loss bug. Output from reproduction script: ✓ Test 1 passed: Normal tool call works correctly ✓ Test 2 passed (bug confirmed): Malformed …