feat: add auto-fixture for Istanbul E2E coverage collection

[gustavolira]

Summary

• Adds a _coverageCollector auto-fixture to the shared test object that automatically collects window.__coverage__ from the browser after each test
• Guarded by E2E_COLLECT_COVERAGE=1 — zero overhead when disabled (no page.evaluate, no fs operations)
• Writes Istanbul coverage JSON files to coverage/istanbul/ (configurable via COVERAGE_OUTPUT_DIR)

Why

The rhdh-plugin-export-overlays repo builds Istanbul-instrumented dynamic plugins and runs E2E tests against them. The instrumented plugins populate window.__coverage__ in the browser, but nothing collects it today.

Since all 18 workspace test files import test from @red-hat-developer-hub/e2e-test-utils/test, adding an auto-fixture here gives every test automatic coverage collection without modifying any test file.

How it works

1. Instrumented plugin loaded in browser → window.__coverage__ populated
2. Test runs and exercises plugin UI
3. _coverageCollector auto-fixture teardown fires (after test, before page closes)
4. page.evaluate() reads __coverage__ from browser
5. JSON written to coverage/istanbul/<testId>-<timestamp>.json
6. Overlay repo's coverage-reporter merges all JSONs → lcov.info
7. upload-coverage.sh sends lcov to Codecov with --slug --sha --flag

Design decisions

• auto: true — runs for every test without test file changes
• scope: "test" — collects per-test, not per-worker
• await use() first — teardown runs AFTER the test but BEFORE page closes (Playwright tears down in reverse dependency order)
• try/catch with empty catch — best-effort; if the page crashed or plugin wasn't instrumented, silently skip
• _ prefix — Playwright convention for internal auto-fixtures
• No new dependencies — uses only node:fs and existing path import

Related PRs

• Instrumentation pipeline: feat: E2E coverage instrumentation with CI pipeline for all dynamic plugins rhdh-plugin-export-overlays#2383
• Codecov flags: feat: add E2E coverage flags for overlay cross-repo uploads rhdh-plugins#2997
• Jira: RHIDP-13411

Test plan

• yarn build succeeds
• yarn typecheck passes
• With E2E_COLLECT_COVERAGE=1 and an instrumented plugin: JSON files appear in coverage/istanbul/
• Without E2E_COLLECT_COVERAGE: no JSON files, no performance impact
• When page crashes mid-test: no error thrown, test result unaffected

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