fix(cluster agent): fix joining KAS proxy subpaths

Description

This change set fixes appending the k8s-proxy subpath to KAS URLs that already contain a URL path.

For example a KAS Url like wss://example.com/-/kubernetes-agent/ is correctly transformed to a proxy path (see unit tests).

Related Issues

Resolves #7441 (closed)

How has this been tested?

• unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap