fix: ensure grid has tabbable elements after it becomes visible

[web-padawan]

Description

Fixes #8098

When the grid is initially hidden, _headerFocusable is set to undefined since there is no first visible row.
Added the call to _resetKeyboardNavigation() on appear and a test to ensure focus gets moved properly.

This makes the grid Tab navigation work in Popover and manually attached Dialog.

Type of change

• Bugfix

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

Hi @web-padawan and @web-padawan, when i performed cherry-pick to this commit to 24.4, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick ecb3ecd
error: could not apply ecb3ecd... fix: ensure grid has tabbable elements after it becomes visible (#8102)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.6.0.alpha3 and is also targeting the upcoming stable 24.6.0 version.