[Index Management] Announce mappings field list changes

[SoniaSanzV]

Closes #242871

Summary

• Add screen-reader announcements for mapping fields list updates in Index Management → Index details → Mappings, including the item count.
• Announcements fire when filtering by Field types and when Search fields results change, using a shared minimal EuiLiveAnnouncer-based hook.

Test plan

• Manual:
  • macOS Sequoia + Safari 18.5 + VoiceOver
  • Change Field types → verify announcement and focus stability
  • Type in Search fields → verify debounced announcement and focus stability

Accessibility notes

• WCAG: 4.1.3 Status Messages (AA)
• Before (per issue): list/table updates after Field types selection but no corresponding SR announcement.
• After: SR announces “{label} list has been updated. {N items}.” for both Field types filtering and Search fields result updates, without forcing focus changes.

[elasticmachine]

🤖 Jobs for this PR can be triggered through checkboxes. 🚧

ℹ️ To trigger the CI, please tick the checkbox below 👇

• Click to trigger kibana-pull-request for this PR!
• Click to trigger kibana-deploy-project-from-pr for this PR!
• Click to trigger kibana-deploy-cloud-from-pr for this PR!
• Click to trigger kibana-entity-store-performance-from-pr for this PR!
• Click to trigger kibana-storybooks-from-pr for this PR!