[Observability 2.0] Adopt EmptyPlaceholder for Data Quality & Alerts empty states

[ShaileshParmar11]

Summary

The Observability 2.0 screens still render legacy empty-state placeholders (ErrorPlaceHolder / FilterTablePlaceHolder). Adopt the shared EmptyPlaceholder core component (@openmetadata/ui-core-components) so the empty states match the 2.0 Release mocks and stay consistent across OSS, Collate, and AI mode.

Scope (OSS-shared components)

• Data Quality → Test Cases (DataQualityTab): blank variant, with a hasActiveFilters prop so copy switches between "No test cases yet" and "No matching test cases".
• Data Quality → Test Suites (TestSuitesTable): blank variant, branching on hasActiveFilters + subTab (Table / Bundle / filtered).
• Alerts (ObservabilityAlertsTable): features variant onboarding (Pick a trigger / Choose the destination / Stay ahead) with a + New Alert CTA gated on create permission.
• New label.* / message.* locale keys added to en-us.json and synced across locales.

These are OSS-shared components, so the change also flows to Collate and AI mode (tracked separately in open-metadata/openmetadata-collate).

Acceptance criteria

• Each screen renders the shared EmptyPlaceholder (icon + title + description; onboarding + CTA for Alerts) per the 2.0 mocks.
• Filtered vs no-data copy differs on screens that support filtering.
• Unit tests cover both the no-data and filtered branches.

Addressed by #30050.