[Security:Assets:Fleet page]Generic error text is announced instead of the one present

[L1nBra]

Description
Errors present for the fields should be correctly announced for the users. Especially for the users using assistive technology, that they understand that error exists and how to fix it.

Preconditions
Security -> Assets -> Fleet page is opened.
Use Screen Reader (NVDA).

Steps to reproduce

1.Navigate to Add fleet server button by pressing Tab key.
2.Press Enter.
3.Navigate to Default namespace section (on Advanced tab).
4.Navigate to the field in this section.
5.Enter any random text (which would result in error).
6.Navigate to the same field.
7.Observe flyout and screen reader.

UI elements + NVDA Speech Viewer

Actual Result

• Error text appears for the field, but is is announced as "invalid entry" for the user.

Expected Result

• Error text is announced for the user: "Namespace is required".

Notes:
Same behavior for same field on Create agent policy flyout (Agent policies tab).

Meta Issue

Kibana Version: 8.17.0-SNAPSHOT

OS: Windows 11 Pro

Browser: Chrome Version 131.0.6778.140 (Official Build) (64-bit)

Screen reader: NVDA

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)
• Understanding SC 1.3.1: Info and Relationships (Level A)(https://www.w3.org/WAI/WCAG22/Understanding/info-and-relationships.html)

Related to: https://github.com/elastic/kibana-team/issues/1280

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)