[EDR Workflows] Fix artifact FTR tests broken by artifacts regrouping

[szwarckonrad]

The artifacts regrouping PR (#257001) reorganized all endpoint artifacts into a unified page with tabs. This changed the page structure so that header-page-title now belongs to the unified "Artifacts" page and is always present, regardless of whether individual tabs have entries.

Previously, trusted_apps_list.ts was skipped (flaky). Our PR #261180 unskipped it after fixing the underlying flakiness — but by that point the regrouping had already landed, making the header-page-title assertions invalid. The test immediately started failing on merge.

artifact_entries_list.ts has the same header-page-title assertions for all artifact types (trusted apps, event filters, blocklist, host isolation exceptions) — these would fail for the same reason once hit.

Fix: Replace missingOrFail('header-page-title') with existOrFail('*-emptyState') in both test files. The empty state test subject is the correct indicator that a tab has no entries.

Closes #261827
Closes #261829
Closes #261833
Closes #261849
Closes #261850

[elasticmachine]

Pinging @elastic/security-defend-workflows (Team:Defend Workflows)

[elasticmachine]

⏳ Build in-progress, with failures

• Buildkite Build
• Commit: 21458b5

Failed CI Steps

• FTR Configs #109
• FTR Configs #109
• FTR Configs #164

Test Failures

• [job] [logs] FTR Configs #109 / Cloud Security Posture - Group 5 (KSPM + Flyouts) Security Network Page - Entity Preview flyout via LOOKUP JOIN (v2) expanded flyout - grouped entities - show host entity details
• [job] [logs] FTR Configs #109 / Cloud Security Posture - Group 5 (KSPM + Flyouts) Security Network Page - Entity Preview flyout via LOOKUP JOIN (v2) expanded flyout - grouped entities - show host entity details
• [job] [logs] FTR Configs #164 / integrations For each artifact list under management When on the Event Filters entries list should be able to add a new Event Filters entry

cc @szwarckonrad