feat: add Slack notification workflow for coverage improvements (#10977)

## Summary

Adds a GitHub Actions workflow + TypeScript script that posts to Slack
when a merged PR improves unit or E2E test coverage.

## Changes

- **What**: New `coverage-slack-notify.yaml` workflow triggered on push
to main. Compares current coverage against previous baselines, generates
Slack Block Kit payload with progress bars and milestone celebrations,
posts to `#p-frontend-automated-testing`.
- **Script**: `scripts/coverage-slack-notify.ts` — parses lcov files,
computes deltas, detects milestone crossings (every 5%), builds Slack
payload. Pure functions exported for testability.
- **Tests**: 26 unit tests in `scripts/coverage-slack-notify.test.ts`
covering all pure functions including edge cases (malformed lcov, exact
boundaries, zero coverage).

### Security hardening
- All `${{ }}` expressions moved from `run:` blocks to `env:` variables
- `SLACK_BOT_TOKEN` passed via env var, not inline
- Unique heredoc delimiter (timestamp-based) prevents payload injection
- `parseInt` fallback (`|| 0`) guards against malformed lcov
- PR regex anchored to first line of commit message

### Robustness
- `continue-on-error: true` on Slack post step (outage does not fail the
job)
- Baseline save guarded by `steps.unit-tests.outcome == success`
(prevents corrupt baselines on test failure)
- Channel ID commented for maintainability
- Top-level `text` field added for Slack mobile push notifications
- Author linked to GitHub profile instead of bare `@username`

## Review Focus

- Workflow step ordering and conditional logic
- Security of expression handling and secret management
- Slack payload structure and Block Kit formatting

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10977-feat-add-Slack-notification-workflow-for-coverage-improvements-33d6d73d3650819c8950f483c83f297c)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

.github/workflows/ci-tests-e2e.yaml

@@ -87,6 +87,7 @@ jobs:
         run: pnpm exec playwright test --project=chromium --shard=${{ matrix.shardIndex }}/${{ matrix.shardTotal }} --reporter=blob
         env:
           PLAYWRIGHT_BLOB_OUTPUT_DIR: ./blob-report
+          COLLECT_COVERAGE: 'true'
 
       - name: Upload blob report
         uses: actions/upload-artifact@v6
@@ -96,6 +97,15 @@ jobs:
           path: blob-report/
           retention-days: 1
 
+      - name: Upload shard coverage data
+        if: always()
+        uses: actions/upload-artifact@v6
+        with:
+          name: e2e-coverage-shard-${{ matrix.shardIndex }}
+          path: coverage/playwright/
+          retention-days: 1
+          if-no-files-found: warn
+
   playwright-tests:
     # Ideally, each shard runs test in 6 minutes, but allow up to 15 minutes
     timeout-minutes: 15