feat(pydantic-ai): 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)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

---

New Features ✨

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

Internal Changes 🔧

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

---

_{🤖 This preview updates automatically when you update the PR.}

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}