Allow saving <button> on menu LinkWidget

3202166-1.patch has been queued for custom testing for 9.2.x

Patch #1, Works fine for me, Adding an after-patch screenshot for reference.
RTBC+1

This could use some test coverage.

Added test for route: as link.

Applied patch #8 and it works fine.

Before patch:

After patch;

RTBC +1

Verified and tested patch #8.
Patch applied successfully and looks good to me.

Testing Steps:
# Goto: admin/structure/menu/item/1/edit?destination=/admin/structure/menu
# Add into link field
# Save it.

Expected Results:
# User should be able to save the created menu with link
# User should see "Menu linked has been saved" message once clicking on the Save button.

Actual Results:
# User is not able to save the created menu with link and able to see an error.

Looks good to me.
Can be a move to RTBC.

This now has tests

There are two places in LinkWidget that reference route:<button> but we're only updating one of them in this patch.

Update as per #14

Applied patch #8 cleanly and working as expected, able to add button in the menu section.
Screenshot for the reference.

Verified and tested patch#15 on the drupal 9.3.x-dev version. Patch applied successfully and looks good to me.adding screenshot for the reference

$this->drupalPostForm() is deprecated.
Replace
$this->drupalPostForm('/entity_test/add', $edit, 'Save');
to

$this->drupalGet('/entity_test/add');
$this->submitForm($edit, 'Save');

Attaching a new patch.

The patch applies cleanly and works as expected.

Just uploading a test-only patch to expose the test coverage. Also added a missing article to the code comment. Leaving RTBC since the change is trivial and I would have fixed it on commit if there had already been a test-only patch tested.

Thanks everyone for fixing and manually testing this change.

I'm removing issue credit for @kleiton_rodrigues, @Rinku Jacob 13, @Madhu kumar, and @chetanbharambe, since your colleagues already manually tested the patch and provided screenshots without any intervening changes to the functionality that was being screenshotted. In the future, it's more helpful if you manually test patches that have not already been tested by someone else (especially someone from the same organization). I did retain credit for @Abhijith S because the "after" illustration added additional information not present in @Gauravmahlawat's screenshots that helped me understand the change. Thanks for your efforts!

It'd help if my test-only patch were not just the same as the actual patch.

OK, that fails as expected. (Although the debug is pretty terrible, "Undefined array key 1". But that's not this test's fault.)

Committed to 9.3.x, and cherry-picked to 9.2.x as a non-disruptive bugfix. Thanks!