[WorkChat] Change index selector to a ComboBox for the "Index Source" integration

[artem-shelkovnikov]

Closes https://github.com/elastic/search-team/issues/9656

Summary

This PR adds changes the input that allows user enter the index when configuring a WorkChat integration with "Index Source".

The video is better than a thousand words:

Before:

Screen.Recording.2025-04-04.at.18.23.52.mov

After:

Screen.Recording.2025-04-11.at.18.26.07.mov

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, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests 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
• 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 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

[pgayvallet]

Functionally, it's looking okay!

Added comments / suggestions on improvements (note: those can be ignored if this is meant to be something temporary. I have no idea how much we're thinking of fully trashing the current UI for integration configurations)

[jedrazb]

LGTM, some nits

[jedrazb]

If we are surfacing the error in a toast, we should wrap the error string with the i18n logic

[artem-shelkovnikov]

I've copied it over from another WorkChat component - none of components are using i18n, so it should be fine to not do it here? Or should I change this everywhere in the code within the PR?

[jedrazb]

i think it should be fine to not worry about i18n at this stage

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: afe2534

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
wciIndexSource	13	16	+3

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
wciIndexSource	20.3KB	21.9KB	+1.6KB

History

• 💚 Build #292291 succeeded bbdec70
• 💚 Build #291974 succeeded f39a8f4
• 💔 Build #291940 failed 0620d01
• 💔 Build #291911 failed 723b745
• 💔 Build #291512 failed a4dc3cb
• 💔 Build #290409 failed 970452f