Tenant UI PR CI cleanups: dedupe lint/build, test workflow concurrency, explicit coverage

Some smaller cleanups for the Tenant UI PR CI, related to recent build speedups (#2071's slow build investigation):

**1. Deduplicate lint/build between workflows**
The `build_ui` action runs native `npm ci` + lint + build before the docker build (which installs/builds again), while `test_tenant_ui.yml` also installs/builds independently. Move lint (and the unused `format:check` script) into `test_tenant_ui.yml` and slim the build action down to checkout → docker build.

**2. Add `concurrency` to `test_tenant_ui.yml`**
`on_pr_opened.yaml` cancels superseded runs on new pushes; the test workflow doesn't, so rapid pushes stack stale test runs. Copy the same `concurrency`/`cancel-in-progress` block.