Report test results & coverage in the CI run summary

[jstayton]

npm test -- --ci --coverage already ran in CI and both reports were already
uploaded as artifacts, so the numbers were only visible by downloading an
artifact or scrolling the job log. This posts them to the run summary, in the
same shape as the jobs in vision-download and
TRUEPIC/webhook-verifier-nodejs#46.

No thresholds. Reported, not enforced.

Matching webhook-verifier-nodejs#46: a separate job, run once on 26.x rather
than across the 22/24/26 matrix so the run gets one summary rather than three,
and set -o pipefail because npm test | tee returns tee's status, not
Jest's — without it the job reports green on a broken suite. Lint & test is
untouched, and the per-Node-version artifacts still come from there, so this
job only writes the summary.

One deliberate difference from #46, which combines the run and the report into
a single step: because the default shell is bash -e, a failing suite aborts
that step before the summary block runs, so no summary is written. Splitting
the report into its own step with if: ${{ !cancelled() }} means it is still
written when tests fail — which is when the test results are the most worth
reading. Verified both ways. #46 has the same gap and may want the same split,
since its spec reporter output is also only in the job log.

Rendered summary:

## Tests

Test Suites: 21 passed, 21 total
Tests:       357 passed, 357 total
Snapshots:   0 total
Time:        1.646 s

## Test coverage

------------------------------------|---------|----------|---------|---------|
File                                | % Stmts | % Branch | % Funcs | % Lines |
------------------------------------|---------|----------|---------|---------|
All files                           |     100 |      100 |     100 |     100 |
...

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