Legend: Series icon button to change color is inaccessible

<!-- Please only use this template for submitting accessibility issues. This is a new feature area for Grafana that we want to improve. We have long way to go to really improve accessibility and would like your help to know where to start. --> **Steps to reproduce**: View a timeseries panel with a legend, and then click the series-icon. At the bottom of this image, it's the green line next to "A series." <img width="752" height="330" alt="Image" src="https://github.com/user-attachments/assets/2f7ce8e7-7fd1-4407-9cf6-294aab28eb37" /> **Actual Result**: The button has no screenreader text, so it **Expected Result**: The button should have some screenreader indication of what it does (pops open the ad-hoc color picker) **Relevant WCAG Criteria:** https://dequeuniversity.com/rules/axe/4.11/aria-command-name **Environment**: - Grafana version: 12.4.0 - Data source type & version: n/a - User OS & Browser: Chrome - Others: n/a