Fix group GET path when sandbox is a subgroup

What does this MR do?

Updates Resource::Group to determine the correct full path when Resource::Sandbox is used as a subgroup.

Closes #331728 (closed)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

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• Tested against Staging:

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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