fix: Fix reasoning_content for openai compatible responses

[Luca Forstner (lforst)]

• Fix missing reasoning text in traces from streamed OpenAI-compatible responses. Previously, reasoning_content fragments were dropped; they are now combined separately from the final answer for each response choice.
• Preserve empty and null reasoning values without allowing later null fragments to erase text already received. This applies to both manual wrapping and automatic instrumentation.