ci(test-optimization): run datadog-plugin-jest unit spec

`packages/datadog-plugin-jest/test/util.spec.js` is matched by
`test:plugins`' glob but no workflow set `PLUGINS=jest` for
`test:plugins:ci`; the existing `integration-jest` matrix runs
`integration-tests/jest/*.spec.js` only, which does not cover the
plugin package's own unit tests. The new joint check flagged it as
never reached.

Add a single `jest:` job invoking the `plugins/test` composite action.
`util.spec.js` is a pure unit test on `getFormattedJestTestParameters`
and `getJestSuitesToRun`, so no service containers or matrix axes are
needed beyond what `plugins/test` already covers (oldest-maintenance
and latest Node).

Author: BridgeAR

.github/workflows/test-optimization.yml

@@ -206,6 +206,19 @@ jobs:
           flags: test-optimization-jest-${{ matrix.version }}-${{ matrix.jest-version }}
           dd_api_key: ${{ steps.dd-sts.outputs.api_key }}
 
+  # Unit tests for `packages/datadog-plugin-jest`. The `integration-jest` matrix above only runs
+  # `integration-tests/jest/*.spec.js`, so this is the only CI invocation whose glob expands to
+  # cover `packages/datadog-plugin-jest/test/util.spec.js`.
+  jest:
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+    env:
+      PLUGINS: jest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: ./.github/actions/plugins/test
+
   integration-cucumber:
     strategy:
       fail-fast: false