…ut (elastic#251656)

## Summary

Adds missing `aria-label` property on the usage of the `EuiFlyout`
component in [flyout component](https://github.com/elastic/kibana/blob/main/x-pack/platform/packages/shared/kbn-elastic-assistant/impl/assistant/common/components/assistant_settings_management/flyout/index.tsx)
causing screen readers to only get the "....you are in a modal dialog..." message read.

See https://eui.elastic.co/docs/components/containers/flyout/#props for `aria-label` usage in the `EuiFlyout` component.

Unit tests added to ensure the `aria-label` property is passed.
Verified using the VoiceOver screen reader on MacOS.

Closes elastic#231501

### Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [ ] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

### Identify risks

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

No risks identified.

*PR developed with Cursor CLI + GPT Codex 5.2 Hight Fast*

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
(cherry picked from commit 7b08c8c)