feat(openai): Set gen_ai.response.finish_reasons on chat completion spans

[manojmulammagari]

Closes #8000
Part of #8004

Summary

• Extracts choice.finish_reason from OpenAI Chat Completion responses (non-streaming, sync streaming, and async streaming) and sets SPANDATA.GEN_AI_RESPONSE_FINISH_REASONS (gen_ai.response.finish_reasons) as a list[str] on both Span and StreamedSpan.
• Filters out intermediate streaming chunks where finish_reason is None and only sets the span attribute when at least one non-null finish reason is present.
• Adds unit test assertions covering single-choice, multi-choice, sync streaming, and async streaming spans.