ci: use public integration test runner

[cesarenaldi]

Summary

• Switch integration tests to arc-public-runner-set-euw1

Verification

• uv sync --locked --all-extras --all-groups
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright
• uv build
• uv run pytest -m "not integration"

---

Note

Low Risk
Only the GitHub Actions runner label changes; no application, auth, or test logic is modified.

Overview
Integration workflow jobs now run on arc-public-runner-set-euw1 instead of arc-runner-set-euw1, routing integration tests to the public ARC runner pool in EU West 1.

^{Reviewed by Cursor Bugbot for commit 5f351c3. Bugbot is set up for automated code reviews on this repo. Configure here.}