Enforce E2E process and port ownership

[isaacroldan]

Why

A failed E2E test can leave a CLI child process or bound worker port that affects the next test.

What

• terminate complete PTY process trees and wait for root and process-group exit
• verify fixed worker ports before use and after fixture cleanup
• preserve test failures when cleanup also fails, with process and port ownership details
• add local coverage for process-tree termination, ownership diagnostics, and worker port allocation

Testing

• pnpm --filter @shopify/e2e lint
• pnpm --filter @shopify/e2e type-check
• pnpm exec playwright test tests/ownership.spec.ts --project=local --repeat-each=10
• pnpm exec playwright test --project=local