feat(openai): Set system instruction attribute for Completions API

[alexander-alderman-webb]

Description

For #5359, it is easier to have distinct handling of the messages and input types for completions and responses APIs, respectively.

Set the system instruction attribute, conforming to OTtel structure for OpenAI Completions functions:

https://opentelemetry.io/docs/specs/semconv/registry/attributes/gen-ai/#gen-ai-system-instructions

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

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New Features ✨

Integrations

• feat(integrations): OpenAI/OpenAI Agents detect and report the time to first token metric (TTFT) as gen_ai.response.time_to_first_token by constantinius in #5348
• feat(integrations): openai-agents streaming support by constantinius in #5291

Other

• feat(anthropic): Set system instruction attribute by alexander-alderman-webb in #5353
• feat(asyncio): Allow to turn task spans off by sentrivana in #5367
• feat(gen_ai): add function set_conversation_id and managing functions on the Scope and apply it on the Span on .finish() by constantinius in #5362
• feat(google-genai): Set system instruction attribute by alexander-alderman-webb in #5354
• feat(langchain): Set system instruction attribute by alexander-alderman-webb in #5357

• feat(openai): Set system instruction attribute for Completions API by alexander-alderman-webb in #5358

• feat(openai-agents): Set system instruction attribute by alexander-alderman-webb in #5355
• feat(pydantic-ai): Set system instruction attribute by alexander-alderman-webb in #5356

Internal Changes 🔧

• ci(release): Fix changelog-preview permissions by BYK in #5368
• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

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