fix: Fix accessibility findings with app layout toolbar

[avinashbot]

Description

Made the following app layout changes:

• Provided a label to the navigation region around the side navigation trigger button
• Removed undefined classname and testid findings
• Provided matching id to bottom split panel to match aria-controls on the split panel trigger
• Miscellaneous dev page labelling fixes

Unlabeled side navigation trigger

The issue we're facing is one of an unlabeled nav element on the page. We can't just assign the ariaLabels.navigation value to it, because we're already assigning to the open side navigation panel, and we can't have two navigation landmarks with the same labels. The "floating button" design doesn't have this issue because it goes away as soon as the panel is opened. We can do the same thing here, by just removing the navigation role around the button container when the panel is open.

I didn't change the nav to a div out of pure caution - didn't want to give this change the chance to break any consumer tests.

Related links, issue #, if available: AWSUI-60014

How has this been tested?

Passing existing tests, verified locally with VoiceOver. Will definitely need running through the dev pipeline (AwsUi-v3-dwaraa) and a dry-run (7134801478), just to be extra sure.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.34%. Comparing base (da7a8fa) to head (4eb02bf).
Report is 6 commits behind head on main.

Additional details and impacted files

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##             main    #3063    +/-   ##
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  Coverage   96.34%   96.34%            
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  Files         779      779            
  Lines       21914    21918     +4     
  Branches     7099     7505   +406     
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+ Hits        21113    21117     +4     
  Misses        794      794            
  Partials        7        7            

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[just-boris]

To make this code more error-proof, it could be a separate object

const navLandmarkAttributes = navigationOpen ? {role: 'presentation'} : {role: 'navigation', 'aria-label': ariaLabels?.navigation};