MudDynamicTabs: Fix disappearing slider with key attribute (#2816)

[gaplin]

Description

Fixes #2816, fixes #8017
I've added key attribute to elements inside the loop responsible for adding panels.
After that slider stays always visible.

How Has This Been Tested?

visually
I've tried to write some unit tests, but I couldn't reproduce the issue using code. Seems like key attribute isn't respected at all in test's context.
I've added example to UnitTests.Viewer just for testing visually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Before

before.mp4

After

after.mp4

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[mikes-gh]

Thanks for the PR Before merging this I want to understand why this bug doesn't show up in the docs.
https://mudblazor.com/components/tabs#simple-dynamic-tabs

[gaplin]

This bug only affects situations when you want to use key attribute to preserve state while closing tab.
In the docs there is no such example.
Here is the one which you've provided but with key attached https://try.mudblazor.com/snippet/caQoabHPVyDzdfkr.
If you navigate to the last tab and then close the first one, slider disappears.

Here is another example from discord's help section:
https://try.mudblazor.com/snippet/wuGyEFxaRSGWtKkL
Without key attribute when you navigate to the last tab then turn the switch and scroll down and then close first tab, the state gets reset but slider is visible.

After adding key https://try.mudblazor.com/snippet/mkwouPnPrzdqaBVU state persists but slider disappears.

[mikes-gh]

@gaplin Thanks for digging into this. We appreciate your PR