Fix projection updates to use current slider time in interactive plot_topomap

[Aniketsy]

This PR fixes #11790 an issue where projection updates in interactive plot_topomap were not using the time selected by the slider.

Changes made

• stored the current slider time index during interaction.
• Used this stored time index when applying projections.
• ensured merged channel information is preserved during redraw.
• cleared stale contour state when projections are reapplied.
• Re-enabled the slider interaction test in test_plot_topomap_basic.

[larsoner]

@Aniketsy CIs complain about changelog update

Do you have a minimal code snippet that you've been testing with that you can copy-paste here so a reviewer can run on main to see the problem, but see it fixed on this PR? That's usually a good way to test locally during development and facilitate review

[Aniketsy]

@larsoner thanks for the suggestion. I don’t currently have a minimal reproducible snippet prepared, but I can go through the changes again and share a snippet to help with the review. I’ll also keep this in mind for future PR's