[APM] Fix fold/unfold button in traces waterfall explorer not clickable

[rmyz]

Summary

Fixes #216790

This PR fixes the z-indexes changed in the emotion migration PR, using the same value (2000) as before.
Also it uses EuiButtonIcon instead of EuiButtonEmpty and aligns its content.

Before

After

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[jennypavlova]

The code LGTM 💯

[kibanamachine]

Starting backport for target branches: 8.x

https://github.com/elastic/kibana/actions/runs/14242793429

[kibanamachine]

💔 All backports failed

Status	Branch	Result
❌	8.x	Backport failed because of merge conflicts

Manual backport

To create the backport manually run:

node scripts/backport --pr 216972

Questions ?

Please refer to the Backport tool documentation

[rmyz]

No need to backport this to 8.x since the error was introduced in this PR and that didn't get merged into 8.x

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 216972 locally

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 216972 locally