[BUGFIX] fix timeseries tooltip hover for stacked and grouped charts

[ibakshay]

Description

This is a port of perses/perses#3574 by @Gavriel-M.
All credit for the original work goes to @Gavriel-M](https://github.com/Gavriel-M)
Also addresses the open review comment from that PR by @AntoineThebaud.

Screenshots

Screen.Recording.2026-07-03.at.23.44.44.mov

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.