[File Explorer Source Control Integration] Accessibility Bug Fixes

[ssparach]

Summary of the pull request

This PR contains accessibility bug fixes for the following issues:

• Section headers 'File Explorer+ Version Control' and 'Add Repositories' are not announced by narrator in 'Windows Customization>> File explorer' page.
• "Error assigning source control provider" is not defined as heading.
• Tooltip is not defined for "More Options" button in Add repository group

References and relevant issues

https://microsoft.visualstudio.com/OS/_workitems/edit/53873821
https://microsoft.visualstudio.com/OS/_workitems/edit/53874085
https://microsoft.visualstudio.com/OS/_workitems/edit/53874117

Detailed description of the pull request / Additional comments

Validation steps performed

Local Build
Unit Tests
Confirmed the tooltip appears for the "More Options" button in Add Repositories UI
Confirmed the following using narrator:

• Narrator announces the section headers 'File Explorer+ Version Control' and 'Add Repositories' in 'Windows Customization>> File explorer' page.
• Narrator conveys the heading level for "Error assigning source control provider"

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated