[RAC] Observability alerts table reads from incorrect indices

[weltenwort]

📝 Summary

The introduction of the correct alias names in #108115 surfaced a problem with the way the timeline and indexFields search strategies construct the indices. This causes the Observability table to remain empty even if alerts are present in the correct indices.

✔️ Proposed solution

• An API exists on the observability plugin that...
  • takes a list of registration contexts and a namespace as arguments
  • returns a list of correctly constructed alias names for the given arguments
• The alerts page uses above API to fetch the list of indices for a hard-coded list of registration contexts.
• The t-grid backing the Observability alerts table receives the index names to query instead of the consumers.

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)