Fix flaky "below the fold" dev-toolbar perf audit e2e test

[eldardada]

Changes

Hardens a flaky e2e test: Dev Toolbar - Audits › can warn about perf issue for below the fold image in absolute container.

The audit gates highlight creation on the image's load event (apps/audit/index.ts awaits load so the above/below-the-fold position checks have accurate layout). The test clicked the audit button and immediately asserted toHaveCount(1) with the default 6s expect timeout. When the optimized image loads slowly (e.g. a contended CI runner), the highlight is not created in time and the assertion intermittently sees 0 highlights, failing every retry.

The test now waits for the image to finish loading before triggering the audit, so the assertion no longer races the image load. No production code changes.

This flaked (all 3 retries) on unrelated CI, e.g. https://github.com/withastro/astro/actions/runs/28129089505/job/83301140796

Testing

• pnpm --filter astro exec playwright test dev-toolbar-audits.test.ts runs 11 passing, executed twice for stability.
• The targeted test passes locally. The change only adds a wait before the audit runs, so it cannot make the assertion less reliable.

Docs

No docs changes; this is test-only. No changeset, since there are no published package changes.

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Latest commit: 93f6618

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[eldardada]

This is a test-only fix for a flaky e2e test (below the fold dev-toolbar perf audit) that fails on unrelated CI runs across the repo. Merging it should reduce spurious CI failures for everyone working on the project.

cc @ematipico - could you take a look when you have a moment?