[Search:Overview] Custom instructions field missing title from announcement

[L1nBra]

Description
Dialog modal, flyout, field visible title should be announced for the users, especially using assistive technology to know what dialog modal, flyout opened, what field is active and what is needed to enter in it.

Preconditions
Search -> Agent Builder -> Elastic AI Agent page.
Use Screen Reader (VoiceOver).

Steps to reproduce

1.Navigate to Edit agent settings and press Enter.
2.Navigate to Custom instructions field.
3.Observe page and screen reader.

UI elements + VoiceOver

Actual Result

• Only placeholder text is announced which is present in the field, but not the title of the field.

Expected Result

• Title of the field "Custom instructions" is also announced.

Meta Issue

Version: 9.4.0

OS: macOS Tahoe

Browser: Chrome Version 141.0.7390.77

Screen reader: VoiceOver

WCAG or Vendor Guidance (optional)

• Understanding SC 4.1.2: Name, Role, Value (Level A)(https://www.w3.org/WAI/WCAG22/Understanding/name-role-value.html).

Related to: https://github.com/elastic/kibana-team/issues/3381

[infra-vault-gh-plugin-prod]

Pinging @elastic/kibana-accessibility (Project:Accessibility)