fix(openai): accumulate streamed refusal and reasoning_content

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Fixes #3785

What changed

On a streamed chat completion, refusal and reasoning_content are now accumulated across chunks, same as content. One change in _response_accumulator.py: both are declared as _StringAccumulator() in the message schema.

Why

Only content, function_call and tool_calls were declared there. Any other string field on the delta went through the new-entry path on the first chunk and the replacement path on every chunk after, so output.value kept only the last fragment. refusal is a first-party delta field, and reasoning_content is what DeepSeek, vLLM and Qwen3 stream through a base_url override. So a streamed trace showed a plausible but wrong value, and didn't match the same call with stream=False.

I kept it to these two fields rather than accumulating every undeclared string. Scalar fields like role can repeat across chunks and would get concatenated.

Trace output

output.value message for the same four streamed chunks (two reasoning_content, two content):

before: {"role": "assistant", "reasoning_content": "is 4.",          "content": "The answer is 4."}
after:  {"role": "assistant", "reasoning_content": "2 plus 2 is 4.", "content": "The answer is 4."}

Tests

New tests/openinference/instrumentation/openai/test_response_accumulator.py:

• content, refusal and reasoning_content streamed over three chunks each come out whole, and role stays "assistant"
• reasoning and content streamed in the same response accumulate independently

On main the refusal and reasoning_content cases fail and content passes. With the fix all 4 pass.

pytest tests   # 488 passed, 1 failed

The one failure is test_tool_calls. It fails the same way on main in a full run and passes alone, so it looks like test-order state and isn't related. mypy is clean on the changed files.

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