fix(ci): use shared Xvfb for parallel Cypress execution
Add shared Xvfb server on display :99 for all three test jobs to fix
display conflicts when running 6 parallel Cypress batches.
Before: Each Cypress instance tried to spawn its own Xvfb, causing
"Display already in use" errors and X11 connection failures.
After: Single Xvfb server shared by all 6 parallel batches per job.
Changes:
- runner: Add shared Xvfb before Stripe/CyberSource tests
- runner_alpha: Add shared Xvfb before adyen/celero/silverflow tests
- runner_v2: Add shared Xvfb before coverage tests
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
