feat(google-genai): Set system instruction attribute

[alexander-alderman-webb]

Description

Set the system instruction attribute, conforming to OTtel structure:

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 ✨

• feat(google-genai): Set system instruction attribute by alexander-alderman-webb in #5354

Internal Changes 🔧

• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

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