[Search:Search Applications:Content page]Active Add selected button with errors in flyout

[L1nBra]

Description
Buttons which save, create indices according to the details filled in form - should not be active when form has errors. Or at least user should be asked to fix errors before proceeding.
Button which does nothing and no announcement about errors when activating it can confuse users. Especially the ones using assistive technology.

Preconditions
Stateful Search Applications -> Content page is opened.
Search application is added.
Use Screen Reader (NVDA).

Steps to reproduce

1.Using only keyboard Navigate to Add new indices button by pressing Tab key.
2.Press Enter.
3.Navigate to Select searchable indices field.
4.Enter any text (f.e.: test).
5.Navigate to Add selected button.
6.Press Enter.
7.Observe screen reader.
8.Press Tab key.
9.Observe the page.

Gif showing issue

UI elements + NVDA Speech Viewer

Actual Result

• Add selected button is active. When pressing Enter on it - nothing happens; no announcement about errors in the flyout; navigation goes to the next element (Cancel button).

Expected Result

• Either due to errors in the flyout Add selected button is inactive
  or
  when user press Enter on Add selected button: focus goes to the field in error (Select searchable indices) and announcement about errors in the flyout is made.

Meta Issue

Kibana Version: 8.17.0-SNAPSHOT

OS: Windows 11 Pro

Browser: Chrome Version 130.0.6723.70 (Official Build) (64-bit)

WCAG or Vendor Guidance (optional)

• Understanding SC 1.3.1: Info and Relationships (Level A)(https://www.w3.org/WAI/WCAG22/Understanding/info-and-relationships.html)
• Understanding SC 3.3.1: Error Identification (Level A)(https://www.w3.org/WAI/WCAG22/Understanding/error-identification.html)
• Understanding SC 2.1.1: Keyboard (Level A)(https://www.w3.org/WAI/WCAG22/Understanding/keyboard.html)

Related to: https://github.com/elastic/search-team/issues/8256

[elasticmachine]

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