docs-infra: run workers tests on PRs (gap let broken deps land)

apex-deploy.yml's test job only triggered on push to rolling, so no
PR-level check ran the workers/ vitest suite — that's how the vitest
3.2/pool-workers 0.18.4 peer conflict landed on rolling undetected
(two dependabot PRs merged clean since nothing exercised `npm ci` in
workers/ pre-merge).

Add a `pull_request` trigger scoped to `workers/**` so the test job
runs pre-merge. Gate deploy-canary and deploy-production with
`if: github.event_name != 'pull_request'` so PR runs test only —
no canary/production deploys fire on a PR (deploy-production would
also auto-skip via `needs: deploy-canary` once canary skips, but the
explicit `if:` keeps intent visible). Concurrency group now branches
per-event so PR runs queue/cancel independently of the rolling deploy
group instead of colliding with it.

Validated with actionlint + a YAML parse check.

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